I had a few problems doing this, and it took a medley of different mediums, and it STILL didn't come out the way I wanted. It's mostly markers, acrylics, and gouache. About 9" by 12".
I had to take a bunch of reference photos... butts are surprisingly hard to draw! The drawback is that now there are currently way too many photos of me in my underwear on my powerbook. It's like a 16 year old's myspace up in my iPhoto app. Speaking of, I should delete those now that this is done...
I'm super bad at composition, so I had to draw a lot of thumbnails, rawr.