place your mat on the floor and unroll it, do not drop it, that huge CRACK! is not cool, ever!!! (for more on cracks see tip #1). If you are taking a hot yoga class try rolling your towel up in your mat when you're preparing your stuff the night before.

when placing your mat look behind you and see if you are blocking anyones view of the mirror, if you are move over slightly if possible.  Respect your fellow students.

if you must be at the front of the class get some proper shorts with a draw string, yoga is about focus but NOT focus on your crack!! If you're looking for that get a plumbing job.