Being tall helps. Sitting in a funny position where there isn't a surface to press your wrists against helps (and may or may not be better for your back, depending exactly how you sit and who you talk to).

Basically, if you're grinding your carpal tunnel against something all day long, you're going to have a problem. If you can actually practice 'correct' typing posture with your wrists up off the desk your chances of avoiding it are much improved. Mice are even worse than keyboards, since the whole concept of them implies dragging your wrist across a surface.

If what really matters is keeping pressure off your wrists, this means you can switch to using your palms for support -- using the ball of your thumb or a point inwards towards the fingers on the heel of the hand. Radio Shack used to sell a great so-called "wrist rest" that was a contoured piece of firm foam-covered plastic specifically contoured to cup that part of your hand; today everything on the market is a gel or foam that seems to encourage the user to squish their wrists down into it -- at which point it'll pack down and be no better than a firm surface anyway.