I recently stumbled across the idea of using a piece of foam/spongy material to add random patterned battle damage/wear and tear to my models. As soon as I read about it I just HAD to give it a try on a model. The first model I used it on was my test piece for my recently decided upon Sons of Medusa, which you can see at the bottom of this post for a full example of how this technique looks.
Step One is to paint your model up to normal standard with your base colors, I have used Space Wolves colors here since I've got a bunch of the Wolves laying about not doing anything!
Step Two starts by finding an appropriate spongy material. For this tutorial I used a small chunk of blister pack foam:Once you've located an appropriate material, rip off a ragged piece like I have above and dip an end into a darker colored paint. For this tutorial I mixed 50/50 Shadow Grey/Chaos Black. Apply the color to the model where damage would normally take place. Don't worry about being too liberal, you can always clean it all back up with the base color to get just the right amount of damage.
Step Three is as simple as highlighting the bottom edges of the damaged areas to add a slight bit of depth to the 2D surface. Here I've used Space Wolves Grey.
You can really go nuts with this technique in both color variation as well as amount of damage you apply. The randomness of the ragged sponge pieces help add realism compared to hand painting it on. I plan on using this on many many more projects in the future. It's just to simple!
What have you used to get realistic battle damage on projects? Feel free to share your ideas and experiences!