To begin, my unit of measurement will be the inch, primarily using inches per 360 degree turn (in/360) as my measurement of sensitivity. If you use metric, an inch is roughly 2.5 centimeters.

Using a gaming mouse is recommended, but not essential to having good aim. In saying that, most gamers consider it a great benefit to their playing, and with mice available at all price ranges, it is a reasonably low investment for a definite and noticeable benefit.

Most mice are advertised with their DPI, this is "dots per inch", and is a measurement of the lasers reading of the surface you are using. Mouse DPI is largely marketing. High DPI mice can lend a greater benefit to high sensitivity players, but do not place DPI as your only priority. When researching your mouse, hunt for any issues with the laser or problems with reliability and build quality. Positive things to look for in the reviews are a good polling rate, comfort and build quality.

Here are some useful links for more reading on the matter:

Techenclave[www.techenclave.com]
http://gamingmouseguide.com/
http://www.head-fi.org/t/412804/all-about-the-gaming-mouse-a-buyers-guide-mouse-fi
http://www.esreality.com/index.php?a=post&id=2024663

If your desk is at all reflective, sticky or glossy then it is recommended that you use a mouse pad for gaming. Glossy or reflective surfaces can cause problems with the laser, and if your surface is slippery it can make your own hand slide across the surface, which is a severe hindrance. In general using a pad is recommended regardless.

What is mouse acceleration? You can find out for yourself by running a server, holding your mouse in one spot, marking that place (or use the edge of your keyboard) and moving your mouse across your desk VERY SLOWLY until your character has done a 360. Now return your mouse to it's original position but move your hand quite quickly. You might notice that when you move your hand fast your character turns further than when you turn slowly. That is mouse acceleration and it is a hindrance to people looking to improve their aim and muscle memory long term. Take note of the rough distance you took to move slowly for later. We'll call this your sensitivity, and when measured can be expressed, for example, as something like my own: 8.75 in/360. If you don't know your sensitivity you can calculate it this with this [www.notalent.org]tool. Almost all high level and competitive players shy away from mouse acceleration and anything that can induce inconsistency into their play.

Windows and Macs are known to include their own acceleration curve integrated into them, so you need to activate raw input and deactivate all acceleration in TF2 to bypass this.

You need a few settings:

m_rawinput "1"
mat_vsync "0"
m_filter "0"
m_customaccel "0"
m_mouseaccel1 "0"
m_mouseaccel2 "0"

You can place these commands in your autoexec.cfg folder, located at "C:\Program Files\Steam\steamapps\(STEAM NAME)\team fortress 2\tf\cfg", or you can place these in console. If you have not yet used console, you can activate it by going into options>keyboard>advanced and enabling developer console, and bringing it up with the ` key above tab there.

What do these do? Raw input attempts to take input directly from the mouse. This is not always featured in other games, hence the fixes coming later. The "m_" commands deactivate acceleration. We will get to vsync soon.

Lastly, launch options. To get these, open your steam library, right click on TF2, select properties and click "set launch options". You can enter the following commands in, separated by spaces:

-noforcemaccel

Prevents TF2 forcing mouse acceleration on.

-noforcemparms

Prevents windows attempting to apply its settings to TF2.

Leave this box open. You might need to use it later.

You will notice a more responsive mouse, and small movements will feel more twitchy and sensitive, whereas larger movements will feel slower and less sensitive. This is simply you being used to mouse acceleration for so long that a direct 1:1 input:movement ratio feels foreign to you. You will get used to it. Also, you may have to readjust your sensitivity to get it back to where it was so you retain consistency, hence the rough measurement we took earlier on.

Vertical sync is where your computer is forced to send frames to the monitor at a certain rate, resulting in less than perfect reactions and can fall out of sync if your framerate falls below your refresh rate. Removing vsync allows frames to be shown on your screen as soon as they are rendered. This can cause tearing if the game and your screen are working at too different rates, so you need to make sure your framerate is at all times above your monitors refresh rate (standard is 60Hz). If you use acceleration, you will need to cap your fps
Code:

fps_max 61

Or for higher end computers/ 120Hz monitors:

fps_max 121

Or even

fps_max 241

The advantage is you can now view your surroundings with more accuracy and faster reactions than before.

If you have mouse acceleration off, then don't use an fps cap to reduce tearing:

fps_max 0

Framerate is the most important aspect to gaining the smooth response required for your aim. If your framerate falls below 60 fps at any point in game, then you need to turn down your graphics. If you're not sure what kind of framerate you're getting, then use the following command in console and play a game to see how it turns out:

showfps

You will gain the most benefit from averaging about 120 fps, primarily with a 120hz monitor. You can improve your fps in a multitude of ways. Firstly, I'd advise you experiment with Chris' FPS configs[fakkelbrigade.eu]. These allow you to get the bare minimum of graphics in ways not possible using the options menu. Secondly, there is the SPUF fps fix. Provided your antivirus is slowing down your game, this will be of greatest benefit without any loss in quality. Lastly there are more launch options. Depending on your current settings, these can also provide a nice boost, possibly without a loss in quality.

Here are the launch options your can use:

Important:

-noipx

Removes an old and unused function.

-dxlevel XX

Sets your DX level. 98 is the most recent support. 81 affords the most performance.

Optional:

-console

Opens the console upon startup.

-refresh XXX

Sets the game's refresh rate. This should be equal to your monitor's refresh rate.

-novid

Gets rid of the video on startup.

More reading on the matter of vsync and framerates can be found here:

http://www.tweakguides.com/Graphics_9.html
http://hardforum.com/showthread.php?t=928593

Note: if you prefer acceleration for media work or general usage, you may find this uncomfortable.

Windows:
Some games do not feature raw input functionality, so it is important that windows is optimized for gaming. Go into the control panel, select mouse and go to the 'pointer options' tab. Place the sensitivity slider on the 6th bar and deactivate 'enhance pointer precision'.

The sensitivity (select pointer speed) slider can effectively cut out movements sent by your mouse. For example, if you place the slider on 3, Windows introduces a multiplier of 0.25, cutting out 3/4 of all information sent by your mouse. Not good at all. Enhance pointer precision introduces a non-linear acceleration curve. Even the few advocates of mouse acceleration would have you remove this.

Next you need all mouse acceleration gone from windows, this can be done with a registry tweak[www.sevenforums.com] in windows 7 or by downloading this [donewmouseaccel.blogspot.co.nz]program for windows XP, 7 and vista (recommended). If you use a Logitech gaming mouse there is a selection to use "Setpoint implementation" in the gaming tab, use this. You will also see "keep mouse acceleration" and "keep mouse speed", remove these. Be sure to add steam to the games list so gaming mode is always activated.

Mac:
There are applications that do this, such as Steermouse [plentycom.jp], but it can be removed by opening your terminal and using the following command lines:

macpro:~ user$ curl -O http://ktwit.net/code/killmouseaccel
macpro:~ user$ chmod +x killmouseaccel
macpro:~ user$ ./killmouseaccel mouse

Since the effects of disabling mouse acceleration are initially uncomfortable and can lead to you recalibrating your aim, now is a perfect time to be sure that your sensitivity is in the ideal range for your play style.

The first key of sensitivity in TF2 is to not fiddle with it. Consistency is key and it is important you practice and become good with the same sensitivity. Some days it might not feel quite right, but you need to carry on through because adjusting it will make you worse in the long term. The second key is that you are where you ultimately feel comfortable playing. In saying this, there is a general consensus amongst the competitive community that the ideal range is between 6 and 12 in/360. Anywhere faster than 6 inches tends to be imprecise and inadequate for aiming and it is recommended you lower it. If it takes more than 12 inches to turn then you may suffer attempting to engage threats from behind or rocket jumping.

Your personal ideal sensitivity will be affected by your personal tendencies and preferred classes. If you tend to use your arm to aim while keeping your wrist relatively straight, a lower sensitivity is good. While if you need to keep your arm still while moving your mouse with your hand or fingers, a higher sensitivity is needed. If you are one type, try experimenting with the other. You might find you can aim better.

For classes: projectile classes and classes that require less combat and a higher requirement of awareness make use of higher sensitivities. Precision is not always a priority when you have explosives, the need to rocket jump, or the need for acute awareness of your surroundings. If priority is placed placed more on aiming, such as many hitscan-reliant classes, then a lower sensitivity is of more benefit.

For this reason people who tend towards Medic, Soldier, Demoman and Engineer do tend towards a higher settings, whereas people who like Scout, Pyro, Heavy, Sniper and Spy may gain more benefit from using a lower sensitivity. While it may seem like classes such as Scout, Heavy or Spy need higher sensitivity because they are often in the middle of the fight, however making a habit of aiming by flicking your mouse is bad, and almost everyone can benefit by steadying their aim, tracking and lining up their shots more carefully and simply taking the extra effort to look around when necessary. In the end, the choice is personal preference, but you may like to experiment by changing your sensitivity and/or the way you move your mouse to see if it helps you with your favourite classes.

By now you should have a rough idea of where you want to be. From here on it is experimentation. If you want to test out your new settings on bots, download a map called tr_walkway[tf2.gamebanana.com].

Now once you have decided on your sensitivity, it is recommended you use a very high DPI setting in your mouse to allow for the greatest precision. If you are a lower sensitivity gamer then using your mouse's highest DPI setting is not as essential. A 3 inch 360 (very high) will need a DPI of a bare minimum of 1,800, so you can see that most mice should be able to make it easily for your sensitivity. If you use a non-gaming mouse then you can use the Enotus mouse tool [www.gaming420.com]to calculate your mouses DPI. Generally the DPI of a low end work mouse is too low and inadequate for gaming. You can re-calculate your sensitivity for changing your DPI with this useful tool[www.users.on.net]. I highly recommend using this to set the max DPI.

Here is some more reading on the matter of DPI and game sensitivity:

http://forum.notebookreview.com/accessories/584013-high-dpi-low-sensitivity-medium-dpi-normal-sensitivity.html
Overclock[www.overclock.net]

It is possible to script multiple sensitivities into classes and even weapons.
It is used by some players, so I will cover the potential advantages and disadvantages of each.

The advantage of sensitivity scripting is that it theoretically allows you to retain that 'ideal' sensitivity for each weapon and gives you short term ease of use. It is beneficial to people who are used to low sensitivities but found a higher one was necessary for TF2. For example, a Spy may wish to use a lower sensitivity for revolver aim, but still requires a higher sensitivity for stabbing. I can recommend this type of scripting for Spy and Sniper only. Scripts to do this can be found online, such as through the wiki or Gamebanana[gamebanana.com]. Arguments against scripting are that when you switch weapons you essentially have to re-calibrate your aim very quickly, so you might misjudge shots more often. Scripting multiple sensitivities can hurt your aim improvement long term. Using the same sensitivity universally allows you to focus on the muscle memory improvement of that one speed, allowing for better aim long term. If you are a previous scripter that finds it hard to return to a single sensitivity, then go for the lower value. It is easier to learn to melee on a lower value than learn to aim on a higher one.

Other types of scripting, such as weapon binds, are fine so long as you don't find it breaking or you having to make more hand movements than previously.

If you like what you saw, then I recommend you go here for some advanced reading and stats on your setup:

http://www.funender.com/quake/mouse/index.html

If you have any questions or improvements, feel free to submit.