Wrist problems have been discussed on this Tribe several times. Just try searching on "wrist" in the All Yoga search box.

Here was my response to a prior thread:

Use the whole hand in contact with the floor. Press with your finger tips, as if you are clawing the floor. That should create a sense of muscular energy drawing from the under side your wrist up toward your shoulders (as opposed to letting the weight just dump into the heals of your hands). Don't overly spread your fingers in a way that does not allow that "clawing" action.

Also, look at your wrist just holding it up (not weight bearing). When you flex your wrist, is there a bulge near the heal of hand? This bulge is the retinaculum pressing outward because of compression in the wrist joint. Now add a little resistence with your other hand pressing on your fingers and the first hand pressing back. See if you can create an action that draws the bulging retinaculum back in. This is creating space in the wrist joint. This is the same action you should try to create in Adho Mukha and anytime the hand is flat on the floor bearing weight.

Another thing to check is the alignment of your shoulders. It quite often is the case that wrist pain can come from bad shoulder alignment. The arms should be straight with the muscles (biceps, triceps, etc.) toned. In Adho Mukha (downward facing dog pose) the entire underside of the arm up to the armpit should feel as if it is resisting up away from the floor even as the heart moves closer to the floor. The arms draw up into the shoulders and the shoulder blades lie flat on the back behind the heart.

There is the feeling of muscular energy always drawing from the hands up into the heart, even as you organically extend from the heart back towards your hands and up into your hips and through your legs, in the full pose.

I would never recommend practicing on "fists", because it is very difficult to keep the wrist stable. Though, practicing occasionally on fingertips or "ridge tops" (lifting the heel of the hand only) is a great way to strengthen the fingers, wrists, and forearms, but it is not practical in all poses and transitions (e.g. vinyasas with jumping, etc.) The goal should be to practice with flat hands pain-free.

I hope that helps.
Saprem,
Scott