Using the sponge technique gives great results, if you're using it with a light hand. Less is more, I think. Emphasise the dents and scratches by painting very thin light lines below the marks, like Stefan says. I have used a torn off piece of blister pack foam with success. Dip it in some dark paint, wipe it off so that it's almost dry (pretty much like drybrushing) and dab the piece carefully on the surface you want to "beat up". Try on something else first, so you get a feeling for how much paint should be on the piece of foam and how distinct the dabbing should be.