As a perfectionist, I've spent hours trying to acquire perfect aimbot-like aim. I am nowhere near that, but there's some stuff I've learned from this journey. I am bored at 2am and have nothing to do, so here's a share of my thoughts and experiences regarding mouse settings and aiming. The most important parts of this guide are "Aiming Tips" and "Configuration & Settings" so if you're not so into reading(Which is really bad dare I say) then just check those two out.
Here's a video first showing my experience:
(Music is Killswitch Engage - This Fire)

Aiming is broken down into two major categories. These are:
1. Arm Aiming: Using your whole arm to move the mouse. Characteristics include lower sensitivity, bigger mousepad. The good thing about this is that you can aim with a lot of precision, but turning around is a hard task.

2. Wrist aiming(My personal recommendation): This is when you have your wrist resting on the mousepad, aiming by only moving your hand. Aiming up and down is done by pulling/pushing the mouse with your fingers. Unlike arm aiming, it isn't that precise, but you have control of all angles. With enough practice, the con of the not so precise aim can be nullified.

Then we've got the ways of holding a mouse:
1. Full-Hand: When your whole hand is covering the mouse. Works well with Arm Aiming and larger mice.

2. Fingers: When just your fingers are on the mouse, with your wrist resting on the mousepad. Pretty common and all around great.

3. Claws(My personal recommendation): Like 2, except you hold the mouse with just the tips of your fingers/claws. Easy aiming up and down, and also the mouse feels lighter. This can be hard to get used to, but imo it's worth it.

Your hardware matters a lot in aiming. Logically, having a better mouse and mousepad will make aiming a whole lot easier which is partially true, but this is not always the case. Higher priced mice do not make you more skillful, as there are mice out there in the price range of 10 euros that beat most Razer mice. Choosing a mouse, these things should be kept in mind, not in order:

It's wired: Wireless mice might be portable and all, but in gaming they are weak as they have really high input lag.

Weight: A mouse should be as light as possible. When a mouse is heavy, aiming accurately is harder since more force is required from your arm in order to be pushed. Adding to the previous part, wireless mice always need to have a battery inside them, which adds like a ton of weight.

Specs: A high polling rate and medium-ish DPI (Around 1000-1500)would be good. If you can go for more than that, go ahead; it won't give you the upper hand or anything but hey it's your life, your choices.

Configurable: Pretty much all mice with the above specs would also come with a configurator, so I guess this isn't that much of a problem.

Fit: The way a mouse fits in your hand is obviously important, find something that's comfortable. You can get used to new mice that feel different.

As for your mousepad - Eh, just about anything that's labelled "for gamers" will do.

Your computer hardware also needs to be great. A better computer gives more fps; which gives faster mouse input.

The main attraction. Here's some stuff I've picked up.

Twitching/Always moving
If you've ever encountered an aimbotter in a game and spectated them in first person view, you'll have noticed that there's some sort of "twitching" going on while aiming at heads. After I've imitated that a few times just for laughs, I've noticed that it improves my aim. The reason is pretty simple:

Take any object that is slightly heavy. Put a little bit of force into it and you'll notice that it won't move until you put some more force into it(I've learned this in school but don't remember what it was called). After you've put enough force to move it, you can reduce(without stopping) the force you're putting to what you had before which wasn't enough to move the object, and it'll still be moving, just slower.

Let's put it in some other way - Objects have a property that we'll call ''Initial Force'' which marks the amount of force needed to begin moving. For example, let's say a box has an Initial Force of 15.
If I push it with a force of 10, it won't move. If I push it with a force of 16, it'll move. While still moving it, I can now reduce the force to 10 and it'll keep moving, but slower.

Let's say that box is the mouse. After you've moved it, if you keep moving it all the time without stopping you could easily have a lot of precision without skipping pixels, allowing easier flickshots(instictivly jumping your crosshair on the target in a split second). This can be done by either twitching, or just always moving your crosshair.

Being Calm
At the heat of a game, you might not notice yourself holding the mouse tighter. Maybe you go on empty servers with a friend to practice and find that you can easily aim on everything, but in actual games you have trouble sometimes. That's because holding the mouse tighter can wreck your aim, something a lot of people do in the middle of a game. Focus on your wrist to keep it relaxed at all times.

Welcome to the most important part after practice. Even if you're used to bad settings (Which at the time you didn't know were bad), switching over to good settings feels like a weight coming off off you.

Windows Mouse Settings
So first, we'll take a look at Windows' Mouse settings, which could be found by searching "mouse" in the Control Panel. (Just so I don't have to go through all Windows versions)

Move over to the Pointer Options tab, and drag the Pointer Speed to the 6th option.(Middle)
Having it lower or higher gives slight inaccuracy. Also, "Enhance Pointer Precision" should be unticked. This is acceleration, and you should be rid of any acceleration.

What acceleration does is basically the faster you move your mouse, the pointer goes further than moving your mouse at a slower speed. So this pretty much prevents any kind of Muscle-Memory(Flicking your mouse quickly, knowing subconciously where it will land).

Mouse configurator
Then fire up your mouse configurator that came with it(If there is one).
Again, you will turn off acceleration, that stuff's no good, really. Set your polling rate and your dpi to the highest. Change the X and Y axis as you like, I have my Y axis much faster than the X axis because this way I can easily aim up and down by pushing/pulling the mouse with the tips of my fingers.

In-Game Settings
These don't apply to just Left 4 Dead 2, but any game with the options available.
Go to Options>Video Settings and turn off Vertical Sync. It's not worth the visual improvement at all, unless you prefer having MAJOR input lag. Also, adjust your graphics so that you can get a good amount of fps(Which you can check with the console command net_graph 1).

I recommend something with an average of at least 80 (Going down to 60, and can go up to 100) but of course the more you can get the better it would be. Set it to the lowest if you have to. This is really important. Higher than 120 fps can give lightning-speed input, which is what you'd love to have.

Now I want you to play around with Multicore Rendering being enabled and disabled; I've noticed aiming feels way better when it's disabled. It's up to you though.
After that, go to Options>Mouse Settings and disable Mouse Filter and Acceleration and enable Raw Mouse Input.

Enter a single player game after you change them to try them out. Use the console command "sensitivity" to change sensitivity.

Inches per 360
This useful tool[www.notalent.org] can help you calculate inches per 360(how many inches you move your mouse in order to complete a 360). Arm Aimers can have pretty high values such as 18-24, while Wrist Aimers can have 4-8.

(Credit to MrKringle for letting me know this is important)

MrKringle: your first reaction and direction you aim for is given by the sound

There's no arguing here, that statement is more than correct. Good sound awareness is like an extra pair of eyes; allowing you to know where your enemies are without really looking at them.

MrKringle: but you forgot the most important thing! the right headset

Like mice, a more expensive headset does not make it more "right". Ever since my headset broke, I've been using earbuds that came with my Samsung phone and have been doing just fine.

Anyway, moving on to some setting tweaks again:
In-Game Settings
Go to Options>Audio and change "Speaker Configuration" to "Headphones" or, even better, use the console command "dsp_enhance_stereo 1". Also, set "Sound Quality" to "Low".

Now put on your headset/headphones/earbuds and pinpointing enemies with sound should come naturally.

READ IF USING LAPTOP:
Or maybe it won't: At some point, I wondered why it was so hard to pinpoint when it was an easy task. Turns out, most laptops have "audio enhancing" programs, namely IDT Audio. Sure, these programs give sound a crispy, HD touch but it messes with stereo or something, turning it into mono. No idea why, but after I went to Control Panel>IDT Audio Control Panel or whatever else you have on yours and disabled it, I could pinpoint with such ease, where previously it was impossible.

Now that your mind's been opened with these configurations and tips, you can keep on doing the most important thing: Practice. Learn from your mistakes, watch players who are better than you either on Youtube or in-game. Don't be ignorant thinking you are the best because there will always be someone better than you. Don't be pissed about that fact; learn from those above you. I personally don't believe in any higher powers, but in people. Meaning that I believe that the goals a person sets can easily be achieved as long as said person believes in themselves. That's why I believe in everyone, and think everyone should believe in themselves. If however you don't, then believe in me who believes in you.