What kind of chair is best to use while working at the computer?

What kind of chair do you think is best to use at home while working at the computer? I'd like to know since I spend a lot of time sitting at my desk, using my laptop. I have an adjustable desk and chair, but what position should they be in? How high should the desk be? How should my chair be adjusted? (It has an adjustable lower back support bar, and it can lean forward or backward) And is there anything I should do to the chair (such as putting a towel or other support behind my back) that might help my posture?

Also, is there any specific kind of chair that is especially good for your back, so that if you're sitting in it for hours, you can maintain healthy posture?

Thanks for any suggestions you can give.