Wearing Vedic Gems

The most important thing is that the gem has skin contact, which means it must be set in a way that the back is exposed. The light must be allowed to shine through the gem onto the skin. In effect, this is like color light therapy.
Gems should be worn close to the skin, exposed to the atmosphere, and if possible, not hidden under dark clothes. Let their radiance shine forth!
It's fine to take off your jewelry at night, just remember to wear it in the morning. Many vedic gem astrology books recommend never taking off the gems; they say if the gems are not worn for a while, they lose their powers. I believe it's more like the gems need to be reacquainted to you like an old friend who hasn't seen you in a bit.
Follow the traditional wearing times and rituals for the planetary gems you choose. The mantra is important in that it programs the gem before you wear it. With the mantra, you are telling the universe why you are wearing the gem and invoking the intercession of the planetary god/powers. Then you have a link to a powerful force which will augment your own. Respect your gems, they are denser and stronger than you ;)