The obvious, Sv_Cheats 1, so you can type bindtoggle o "mat_fullbright" in the console, so you'll be able to toggle fullbright on and off.

Then there's some important ones for shadows:
mat_slopescaledepthbias_shadowmap 2 (default 16)
mat_depthbias_shadowmap 0.00001 (default 0.005)
r_projectedtexture_filter 0.2 (default varies, I tend to use this for sharper shadow edges)
and most importantly:
r_flashlightdepthres 2048/4096
This decides the resolution the game gives your shadows, and is very important for shadows.
Darkomni on facepunch made a good tutorial for shadow stuff, and he can explain it much better than I, here's a link to his tutorial.
http://facepunch.com/showthread.php?t=1112981

Now that we've gotten console commands out of the way, onto the ingame stuff.

First off, you'll want to pose your model, once it's posed, it should look like this:
(I pose in fullbright, for better performance and so that I can see what I'm doing)




























After this you'll want to place two highlight lamps, usually light blue/cyan.
Here's the values I'd suggest
Colours can change, it's really up to the author.











































The lamps should be placed like this:
Basically, behind the subject in the picture, on each flank.


And here's how the picture looks by now:


























Pretty alright, innit?
We'll still have to place front highlight and fill lamps, however.

For these, you'll want a weak light, usually in a contrasting manner to the back highlights, so for this one, we'll go for a red fill light, with these values(again, modify in a manner that pleases you):












































And you'll want to place the lamp in a manner that usually is opposite the highlight lamp it's facing, so for this one you'd want to place it like this:












Let's see how it looks:

























Not bad, not bad!
But we'll still need the front highlight, to complete the light setup and give the picture more depth!

This one's the most simple part for me, it's basically just placing a lamp with similar options to the first one(usually around 1.0 instead of 1.25, and with basically no saturation on the color options), and placing it so it gets a somewhat overhead look onto the model.













And here's how it looks ingame:





























Very good!
positions and strengths may be tweaked, all after the author's desires, but for most people this would be it!

Onto color modification and bloom!

So, you want to add more of a punch to your image's color and look?
Well look no further, because I've got you covered!
Well, in the way I do these things, anyway.

At any rate, first off, don't use any of the default setups on bloom or color modification, those all look bad.

Instead, use options similar to these:











Which will give the image this look:





























Other than that, the only thing you'd want to pull after this is SDOF, but that's pretty self explanatory, however, I will refer to a good guide on deviantart, in case any of you good people are stumped on the matter.


Thanks for reading this tutorial, and whoever gets their lighting and post processing options in order after this, you're welcome!

Contact me on deviantart if you have further questions:
http://superninjanub.deviantart.com/

Deviantart SDOF tutorial:
http://forthegmodders.deviantart.com/art/How-to-use-Super-DoF-in-Gmod-152701658