Use the black cloth that came with your MBP. Take it to a sink and thoroughly rinse it under the tap. You need to make sure any dirt, sand, or dust is washed off the cloth, or else you risk scratching something. Get it nice and soaking wet. Then squeeze out the excess water until it's just damp. Wipe gently in a straight line across the screen. Don't obsess too much--it'll only get dusty again later. Too much pressure or moisture is bad. There is no need to wipe with a dry cloth.
If you're really worried about hard water spots, use distilled (demineralized) water.
Do this about once every few weeks, depending on how much you use your Mac. Make sure you get the palm rests, trackpad, and case. The salts in the sweat from your hands, combined with the metal casing, and a bit of electricity (from the fact that when you're using the ungrounded 2-prong AC adapter, the casing conducts a little bit of electricity) can cause tiny black pockmarks to form. Trust me, my old 12" PowerBook G4 has a bunch of corrosion spots on the palm rests. It's ugly, makes the machine look dirty, and worst of all, it cannot be washed off--the aluminum has been etched away.
Don't use any solvents; you shouldn't have to, anyway. Solvents are only good for stains, and you wouldn't do anything to stain the screen, would you? Like dropping pizza, spilling coffee, or smoking right on top of your machine? If you do, then a reconsideration of your priorities might be in order.
I also recommend a silicone keyboard cover, like the iSkin. It prevents the paint from wearing off the keycaps, stops dirt and grime from getting under the keyboard, and blocks any spills (but again, you wouldn't let liquids get near your machine, would you?). Some people say that the cover gets yellow and dirty with age, but that's what it's there for--whatever happens to the cover doesn't happen to the keyboard.
