Unplug your keyboard and clean the tops of the keys and the rest of the outside surfaces with an alcohol-moistened wipe. You can also use mild soap and water (or just plain water) with a damp rag or paper towel. Stay away from commercial cleaners, they tend to leave a residue that attracts dirt.
If you have a small computer vacuum or a regular vacuum with a soft brush attachment or crevice tool, suck up any debris beneath and between the keys. If you don't, no problem; turning the keyboard upside down and blasting it with compressed air is just as effective, if not more so. Use the plastic tube that comes with the compressed air can to direct a series of short blasts between the keys. You could also reverse the vacuum and use the crevice tool to create a very powerful, non-condensed stream of air.
If you have a key that sticks, try pulling it off with your fingers. If you can't, insert a pair of blunt, nonmetallic objects like clean, dry Popsicle sticks on either side of the key and gently lift it off its mount. Apply even pressure to both sides of the key as you lift. Take care not to loosen or misplace any springs or grommets that may be on the underside of the key, and keep the cleaning solution away from the contacts under the key.
In some dire cases, say, after spilling a can of soda or dribbling your ice-cream sundae on your keyboard, you can remove all the keys to clean the keyboard underneath. You can wash the keys themselves in mild soap and water. Of course, make sure they're all completely dry before you re-assemble the keyboard.