One of my favorite yoga bloggers just posted an excellent perspective on why you might be slipping around on your yoga mat in your practice.
I teach hot yoga classes and this is an issue that comes up a lot! Sometimes it is the mat that's slippery, but sometimes it's actually you that's slippery one! If you've got a quality mat and you're still lacking stability in your postures, Mr. Om Shanti addresses how to check the execution of your asana to help correct your slipperiness.
To his list I'll add Vrksasana (Tree Pose). Some people will try to hike up their pants leg to solve slippery pants syndrome. Of course, if you're sweating a lot your bare leg might also exhibit slippery leg syndrome. Here's how to correct this problem without fretting about your outfit!
First, take a nice solid Tadasana. Take a few breaths and connect to your inner stability. Grow roots through your standing foot and allow your other foot to become light. Trace your leg with your hand from your thigh towards your shin and place the sole of the foot on the inner thigh (or shin). Now, focus on strongly squeezing your inner thighs together. This action will result in your lifted foot pressing into the thigh as the thigh presses back into the foot with equal strength. Your foot should stay in place! Now that you don't have to worry about that wandering foot, tuck the tailbone towards the ground, lift the heart towards the sky and spread your arms wide and high, celebrating your newfound stability in vrksasana!