Seeing as a couple of you asked, I thought I'd share my preferred method of painting battle damage.
It's pretty quick and looks decent on the table-top...basically, it won't win you a Golden Demon trophy!
So, this is my ogryn that I'm currently painting, I'll demonstrate my process on his left shoulder pad-
Step 1- a base colour, i this case adeptus battlegrey, followed by a badab black wash
Step 2- highlight the edges of the armour and add chips, scratches with a reeeeeeally fine paintbrush. I find that thinning the paint a little with water gives me more control over where the paint goes.
Step 3- On the larger dents and scratches, put a dab of a darker colour (here, adeptus battlegrey and black 1:1) in the centre of the highlighted parts, leaving a fine edge of highlight around it.
Step 4- On extreme raised surfaces, like the bolts and pointy edges here, I use a little boltgun metal to make it look really scratched.
And on the buzzsaw, I've done the above 4 steps but put a dot of boltgun metal inside the darker colour (step 3) so it looks like the paint's been scratched right down.
So that's just what I do, please feel free to expand on what I've put down here.
BTW- this is a pretty large model, hence the number of steps. On my smaller dudes I usually skip step 3, as the scratches aren't big enough to warrant doing it. And also, on darker armour I tend to do a lot more scratches straight away in boltgun metal, it catches the eye and saves time.
Hope that helps!