How to help drainage of fluid in a joint? (My knee)?

Two months ago I smacked my knee with my skateboard, but since I'm a skater I really thought nothing of it. It swelled up a bit and hurt for a week, but after the briuse healed, there was a small amount of fluid left over in my knee. I went to the doctor, they x-rayed it, did some tests, then they told me nothing is wrong with my knee and I had to wait for the fluid to drain naturally, and that I could go back to skating at my own tolerance. It doesn't hurt unless I completely bend my knee, and it still works comepletely fine execpt for the restriction of movement from the fluid. After two months my knee really hasn't shown any signs of drainage, and it's gotten pretty annoying not having full movement of my knee and not being able to skate at my full potential. Does anybody know how to help speed up the drainage?