Saying that Ashtanga, Bikram, and Vinyasa yoga classes are sweaty is a bit of an understatement. The room is heated (in Bikram can be over 100°F), and you're moving vigorously, so your body sweats profusely to cool you down. In the first 15 minutes, my entire outfit is soaking wet and little beads of sweat start dripping from my chin. The worst part is that my palms get really sweaty too.
That's great for your body, but not for your practice. There's nothing I hate more than trying to do Down Dog, while my hands and feet are slowly slipping away from each other. You can spend all your time trying not to end up lying flat on your belly, that you can't concentrate on following your breath. I have a solution.
You should try a cotton rug. You can continue using your regular sticky mat, you just lay the rug right over it. It makes a wonderful cushioned surface, which is awesome for poses where your bones are pressing into your mat. Rugs are 100% cotton, which means they are washable and biodegradable. Plus they come in tons of colors. Here are my favorites from Barefoot Yoga Company.
Fit's Tips: After a sweaty practice, be sure to let your rug completely dry out. Hang it over a door, because if it remains wet, it can start to grow mold which is really disgusting. Pop your rug in the washing machine at least every 3 practices, if not more.