In general, my advice is to go easy with chipping. It is fun to try and looks good, but not really very realistic many of the times. Look at real tanks (www.Primeportal.com, etc) and you will see that chipping on the modern stuff is really minimal. To add interest go for dirt, oil stains, paint wear, etc. Of course some chips are ok, but it is often overdone. Try to imagine how each chip was made before you place it (ex: bullet hit, rock, etc). Tone them down afterwards. Add other effects to break them up.
To actually make them, I use a small brush and thinned enamel paint that you can remove off the acrylic base if overdone.
Just my 2 cents. Dmitry
PS: the MiG article posted above is very GOOD.