I had to buy pillows for my office and dining room chairs.
Someone said a while ago, I think in the exercise forum, that you could build up your butt muscles to improve that. So I'm working extra on the squats and lunges. Look for exercises for "glutes".
Here are some, or you can look around in the Exercise forum.
http://exercise.about.com/od/butthipsthighs/tp/butt.htm
http://www.weight-lifting-workout-ro...-exercise.html
I really like the side lunges they show on the second site. The Jillian Michaels 30 Day Shred video has some of those and you really feel them working. Also, the elliptical machine set at high resistance is very good for strengthening butt and thighs, especially if you do it backwards (I do 1 minute of every 5 minutes backwards). Your butt will not get bigger, I promise, although it may change shape a bit.