I will start with the tools i use, and then i will write about using both for get good skins.
Desaturate:
Easy, if you do this there will be no more colors, and dont say that it has white-balck, those are tones, not colors.

Posterize:
Not so much does, but is does a great change because it seems that separates the colors, till it looks really weird, so i dont use this.

Curves:
Value, red, green, blue and alpha can be edited here, you can take the line form anywhere, or removing the alpha temporaly.

Levels:
Very useful, lowering the level of a stormie looks really great, and can be used along with another tool ahead...

Threshold:
Nothing great here, just black and white.

Brightness and contrast:
I dont use this much, it result may be a bit more dark than the levels tool, thats everything.

Coloring:
Really clean textures, usfull with levels, if you keep reading you may know why...

Hue and Saturation:
Good combining of this tool can get you another good texture, if you are patient and dont play with it.

Color balance:
Some times, Painty skins, some times, nice skins.

Inverting... but actually inverting:
Both invert, and invert value can get you good skins... i know that the example doesnt looks great, but i wont post the skins
that looks REALLY cool doing that.

The part everyone was expecting...
Desaturate+Levels+Coloring:
Fist of, desaturate the texture, low levels, thats how i did my shadow trooper (black) use colorize to give color to it, or
you can use my Clone parts that has 224 separated TGAs that belong to many clone skins, and it may have color, and a great
texture.
Or better, chose first the color and dont use desaturate, sometimes it make it worse.

Curves+Alpha:
Add alpha or manually remove it, if the skin has alpha, you cant just take down the alpha or you may get a really weird
with-like skin, so, open Curves, select alpha, and move the line to the right.

Over-skin:
Simple, just take a skin, and put it over another one, it must be the same texture class, or you will get a freak in game...
So in layers, there is a opacity scale, if you low it, you are going to start seeing the skin below the first one.
Two skins

Three skins

Layers:
First, just create a layer, do whatever you want with it, it wont affect the texture intill the saving, when you are done,
in brushes, you may find some irregular brushes, select those and select eraser, and use it.not thet you can see now the
stock texture, you are just affecting the second layer, thats fisrt level dirting.

Patience:
Dont hurry, if you hurry you wont get anything as good if you think about it, and give it time.
"You dont have to arrive first, you have to know how to arrive"
My basical skining incules:
BF2
BF2 mod tools
GIMP
MHS viewer
Clone parts
Also, you may think that some skins in the examples are good, dont you ? Make your own, and dont steal, please, its easy.
Credit for tut:
Labj - Writen tutorial
501st_commader - Asking me to post
Lucas Arts - Game
Pandemic - Game and mod tools
The GIMP Development Team - For releasing GIMP
For more information come here:GIMP