A lot of it depends on genetic predisposition, but to some degree you can influence facial hair growth. The male hormone Testosterone is one of the things that influence facial hair, so if you have higher levels of it, you will tend to have more facial hair. About the only way to really increase it is to go on hormone therapy, and I don't know if many doctors will prescribe testosterone injections just so you can grow a better beard, but who knows, you could always ask.
Shaving regualrly (with hot water, soap, and a straight razor), every day or two, may eventually make your beard look fuller as well, because the irritation that shaving causes makes the hairs grow in darker and coarser than they were before... but it becomes a waiting game because in order to get your beard to grow thicker you have to shave it off every day :P
I've heard of some people having success using Rogaine on their face, but I'm not actually sure if that works, and it might do more harm than good haha.
Also, age makes a big difference. As you get older, your beard will probably be thicker... some guys can grow a full beard at the age of 14, others have to wait until they are about 30 for it to come in thickly, so that might make a difference for you too.
I heard that the more you shave the more it will grow faster but I think this is just a myth. Some people have tried laser therapy combined with hair loss treatments like Rogaine to enhance the growth although I think the best thing to do is to wait for it grow.