I agree with the previous post. I was recently doing the challenge myself and did a little bit of experimenting to find what worked best for me. Despite the heat (which actually becomes quite comforting), I found that I preferred capris or pants. My legs were sweating so much and so slick that some of the poses were actually very difficult for me the few times I wore shorts because my hands would lose their grip.
As for tops, I really really recommend the Gabrielle Reece brand work-out tops that are available at WalMart. The Danskin ones that are sold right along side are nice, too, but it was my finding that the GR ones wicked away the sweat better than anything.
I'm assuming you a bit of the ways into your challenge, but I'd still like to wish you the best of luck! It is such an awesome thing that you can do for yourself. I hyper-extended my knee and have had to take a little break, but I'm chomping at the bit to be able to go back to my classes. have fun!
Clothing recommendations for Bikram yoga
... BUT I don't like wearing shorts so I wear capris, and a tightish tank top, don't wear anything too loose because when you bend over it will fall in your face or wherever. Do NOT wear cotton, wear the wick away clothing. 
