Strip the pistol down to the frame and use masking tape to cover all surfaces of the handle and magazine well.
Scratch up the surfaces in the void in the handle behind the magazine well and fill it with Brownell's Acraglas bedding compound. I prefer the runnier red-box mixture to fill this space.
After letting the compound harden, carefully sand the bump on the back of the grip until it is flush with the rest of the grip or until it is most comfortable for you. A small vertical belt sander works best for this. Be careful how much plastic you take off. When the back is ground down to a straight, flush grip, there will be very little plastic left in the middle of the handle. (This is why you fill the space with the bedding compound—as reinforcement.)
Deal with the finger grooves on the front of the grip. The grooves are more aesthetic than practical, as they actually inhibit a proper tight grip. Use the belt sander and/or a sanding drum on a Dremel to carefully grind the bumps down flush with the rest of the handle.
The last bit of grinding you need to do is on the underside of the trigger guard. Use a sanding drum in a Dremel to slightly sand out some of the plastic where the trigger guard meets the handle. This allows your hand to ride up slightly higher on the gun, giving you a more stable shooting platform and faster recovery from recoil.
For the final treatment, you need a hobby wood-burning iron with a chisel-type tip. With the iron heated, carefully press the tip into the plastic of the handle repeatedly to cover it with stipples. Be careful not to push too deep through the plastic. It helps to practice on a toy gun to get the feel of it, but the plastic of a toy gun is much softer, so if you get that down, you're ready for the real thing. Stippling provides a superb grip on the pistol, even in wet conditions.