I used to have the same problem. The best thing I found to do was to shave everyday for a long long time and it eventually came in much thicker and wasn't patchy in the spots that used to be patchy. That helped me out and made it much more full.
It is hard to come up with any solid evidence to prove that there is any way to encourage hair growth. The only little trick I have heard of doing is putting mustard on the area, but otherwise it is completely genetic.
