I don't know about you guys, but my fanny gets pretty chilly during these winter months -- especially now that yoga pants are my usual attire. I had an idea to solve this problem a little while back and here's my solution. I've been collecting soft, slightly worn cashmere sweaters from Goodwill for a few months. I especially love the ones that someone accidentally washed so they are thicker and really super duper soft. Then I just cut the lower half off and sewed the sides together to make it snug. Pulls on over pants and fits tightly.
Here's a pic. And directions. Enjoy a warmer tush soon! :)