Ursula

Well, you could try an exercise ball that's bigger than normal, but if the desk's too tall, then what you really need is a stool-chair--one with an attached footrest. If you're sitting too low to the desk, you'll hurt your wrists, but if your feet are dangling above the floor, then your legs will fall asleep.

Ever considered a keyboard tray? Though they range from flimsy cheap to insanely expensive, you can probably nab one on craigslist that would work for at least a trial...