Ok, newbie question #2 for you guys who answered my sandbags question. How do you guys get those dark creases and shadows on your troops uniforms?

North

One way is with a wash. A wash is a thinned down mix of paint and thinner. I use tamiya paints and water for my washes. I tend to do a very thin wash one or two drops paint to about 5-7 drops water. I opt for more coats to get the desired affect.
Another tip is to use a darkened shade of the base color. That will complement the base color.

So I mix my Tamiya's with water and just paint normally? Or like drybrushing?

You can do a wash one of two ways. The first is to paint into recesses ONLY, and the other is to paint everything and wipe up any excess wash you have left.

As far as painting goes, what Scott mentioned with the darkened base is you paint your base and let dry for a day or two. Then paint the darkened base into the creases and folds. Let this dry for about 20-30 min. Then take a clean brush and put a little thinner on it (VERY little) and blend the darkened shade into your base. Do this to your liking.

Got it, Thanks fellas.

Scott, thanks! Great tips. I was just getting ready to make a post just like this one.

I've used oils and enamels almost exclusively for washes. I have never had any real success with acrylics, thiugh I have been pleased withthe experiments I've tried with Andres/Vallejo. I don't wipe off a wash, but reather let it tone the whole area. For really tight areas, such as fringe, I'll put a bit of the thinned mixture right in the crease. To want to avoid a harsh line between the shades as much as possible. I've found that appliying the drop of thinned oil along pockets and other raised detail makes a faded line by itself.
Remember,the better the figure you're working with, easier this work is going to be.