Which Planets Should You Wear Gems For?

You want to always strengthen your good points and not your bad points. Same thing goes for Jyotish gems - you should wear gems which augment your good planets. Wearing a gem for debilitated planets will probably make them worse as you can't move the planets already affixed in your chart by wearing their gems - after all, you can't change the way you were born. Instead, if you need to pacify weak or debilitated planets, you should donate those planets' gems, not wear them.
A good idea is to donate gems to charity auctions. If you are a jewelry designer, this could be to your advantage - mitigating harmful planets, giving to charity, and getting your name out there at the same time!

References:
http://www.richardshawbrown.com/mysticarticles/standardize.html
http://www.astrologerpanditji.com/page1550.htm