I am usually the last guy to tell you to spend a lot of money on your yoga practice. Some of you have already learned that eventually your cheaper yoga mats will wear out and you'll have to get a new one. Some of them are so cheap that they are also rather poor quality and don't really cushion your knees.
THEN I go and tell you I buy my mats from Manduka.com for $129. I learned quickly that my Ashtanga practice was going to chew up a cheap mat every week. There's a 10 year warranty on these mats if you really do wear them out fast and I actually did already replace the first one I ever bought for free due to the warranty.
They have other yoga mats to choose from but the Manduka PRO mat is by far the most solid and durable mat which has a 6mm thickness to it for help with sore knees. There is currently a 15% off coupon with code ASANA15 for this website right now.
I am NOT telling you that you have to buy a mat like this for my classes or any classes. What I am saying is that if you feel your practice is going to consistently evolve, it's worth considering an investment into a better yoga mat. One that is solid and durable so you don't have to go buy a new one every couple of months. It's YOUR PRACTICE after all, so it's OK to treat yourself.
Thanks for your practice, see you this Saturday for Beginner Yoga at 11:30am!