An example of face paint

This is for those of you who want to make your face blend in with your environment as well as just your camouflage clothes.

[edit] Steps

Get some face paint. It can be any face paint as long as it has colors that match that of your environment. You can use wet ashes for black
Pick the colors you'll need to blend in to where you will be hiding. In forests use green, black, and brown, in deserts use tan, brown and light green and in snow; use grey, green and white, in an urban environment use brown, black and grey.
Cover your face in a "primer" of paint so that if your face rubs on something you will still have the main color. For the primer use the color that best fits the environment.
Now add dark colors to the shiny areas of the face: forehead, cheeks and nose.
Add light colors to the dark areas of the face: under the eyes, under your nose and under your chin.
Once this is done paint a disruptive pattern on your face such as somewhat vertical or horizontal lines.

[edit] Tips

Have a partner or a mirror to help you out.
Camouflage your lips, they're pink!
If you plan on going in the water (amphibious operations) get grease face paint so that it won't come off.
Practice in front of a mirror a few times before you do it for real so that you know what you're doing.
Camouflage paint sticks can get very hard. You can soften them with insect repellent and keep bugs off at the same time.
Avoid creating a pattern. Remember, the point is not to look "cool" but to hide your face. Warpaint is for attention, camouflage is for blending.