You can actually get a rather nice effect doing bright, sci-fi style armour then hitting it with a few weathering techniques. It's not a Tau, but this guy illustrates the techniques/effects you can get and how they look on something with a similarly clean and flat design:
This was painted in a clean-lined, bright and high contrast scheme like you often see on Tau suits, before I applaud some simple sponge weathering and gave it much more gritty, realistic look.
Sponge weathering is simple. Take a piece of sponge or scouring pad, and dip it into a small amount of the colour you want to do chips in, then wipe away most of the paint as you would with a drybrush, before dabbing it onto the areas you want damage. You'll get a variety of sizes/shapes so the damage looks natural, and often smaller/more realistic chips than you would painting one at a time.
Beyond that, any other weathering technique will work just as well on Tau as anything else so look up weathering powders, chipping, sculpting damage and that sort of thing.
Hope that's of some use!