Keyboards can become dirty after periods of extended use, especially if users commonly eat or smoke near the computer. If a keyboard becomes dirty enough, its performance can be affected. Common problems include dead or stuck keys and repeated characters during typing.

WARNING! TAKING YOUR KEYBOARD APART WILL MOST PROBABLY VOID THE WARRANTY OF IT!

[edit] Steps

Shut down the computer and detach the keyboard connector before any cleaning procedure. Do not remove or connect a keyboard while the computer is running. Doing so may damage the machine, especially a non-USB based keyboard.
For a quick cleaning, turn the keyboard upside down and use a can of compressed air to blow out any foreign matter. Be sure to do this in a location where falling debris can be cleaned up easily. Turn the keyboard upside down and tap on it a few times. You should see a bit of dirt fall out. Change the angle and tap harder to make most of it get out.
Clean the sides of the keys with a cotton swab dipped in isopropyl alcohol.
For a more thorough cleaning, remove all of the keys. Do this by gently prying up each key with a small screwdriver or a similar lever. When the keys have been removed, blow out any debris with compressed air. With a moist (but not wet) cloth, lightly swab all surfaces. Do not allow any type of fluid to enter the keyboard.
Clean the individual keys and place them back in the keyboard.

[edit]Alternate Method

Switch off the machine and disconnect the keyboard.
Turn the keyboard upside down and remove all the screws.
Lift the top half of the keyboard (The one with the keys on it) off and put the bottom half to one side.
- There may be some clips on the keyboard, also, check for screws hidden under labels.
Turn the top half around so that you can see the backs of the keys, squeeze the tabs on each key to remove it, the space-bar will have a metal rod, this may be a pain to refit, but is not impossible.
Fill a bowl with hot soapy water (Fairy Liquid works well)
Dump the keys into the bowl and scrub with a brush.
Remove the keys and leave to dry or dry with a hair dryer.
Place the empty top half of the keyboard into the bowl and scrub until clean.
Once everything is dry, reassemble the keyboard.
Press both halves of the keyboard firmly together, if you miss the clips in the middle your keys wont reach the circuits and wont spring up.
Plug in your keyboard, switch on your PC, enjoy.

[edit] Very Quick Fix

For dust and small particles obstructing key movement.
Are you sitting at your desk, right ?
Move your chair off your desk a bit, turn your keyboard upside down holding it with both hands and ... hit your thighs with it, so the keys will get urgent push. You will definitely see many foreign objects coming off. I did it many times with a good result and NEVER spoiled anything (just made my denims dirty...) Obviously it's not the way for laptops.

[edit] Tips

Some people actually wash the keyboard in a dishwasher. However, this is not widely recommended unless you're willing to risk losing the keyboard. The most important thing is to make sure the keyboard is completely dry before using it. This should definitely be avoided with wireless keyboards.
A good idea is to not take off the space bar, it is harder to get back on and easier to break.
If you forget the order in which the keys on your keyboard go, simply boot up your computer. Go to Start>All Programs>Accessories>Accessibility>On Screen Keyboard This will give you a keyboard on the screen, which you can simply copy the order of letters/numbers/characters off.
Easier: just take a picture of the keyboard with your digital camera before you start pulling the keycaps off.
Laptop keyboards are a bit different. It is not usually necessary to remove keys from a laptop in order to clean it. Isopropyl alcohol and Q-Tips or compressed air is usually enough.
The keys on a laptop can be difficult to replace, especially keys like the space bar and the Enter key, which have a seperate support under them that needs to be installed along with the key.
If you remove the keys, be sure you know where each one goes. Make a diagram, or print out this page and use it as a reference.
If your keyboard has keys not shown in this diagram, make a note of their location.
An alternative to compressed air is to use an electric hair dryer with the blower on high. It matters little if the heat is on or off. It's great for dusting too.

[edit] Warnings

When spraying compressed air, do not turn the can upside down. Doing so can cause liquid propellant to spray into the keyboard, damaging it. Also, be sure to use the compressed air in a well ventilated area.
If you remove the keys, keep them away from small children or pets. Loose keys pose a choking hazard.
Do not inhale the contents of the canister. This is a gas substance and can seriously hurt, or even kill you.