A lot of keyboards are higher in the back than the front. In actuality, a negative tilt--higher in the front than the back--may be more comfortable in the long run and possibly help prevent typing-related repetitive strain injuries (RSI). It may be worth trying if you've had trouble with your wrists in the past or are just looking to take preventative measures.
Since it's hard to find keyboards built that way (are there any?), you'll have to grab a keyboard tray in order to achieve the negative tilt position. Look for one that has a ridge of some sort in the back (to keep the keyboard from sliding off) and doesn't bump your knees.
For more information on keyboard trays, check this article on keyboard trays.