Several years? Buy a new keyboard. They aren’t that expensive.
Otherwise, just turn it upside down, and shake it.

Remove all the keys and clean them one by one. De-filth the rest with compressed air.

- Take a picture of your keyboard, or find a photo of the same make/model at Google Images and print it.

- Pry-off each key, using something with a blunt edge. Be careful of the metal bars beneath your space key.

- You might be disgusted by what you find in the recessed bases – dust, grime, crumbs, spilled beverages, animal hair, etc. Using the dusting attachment on your vacuum cleaner, vacuum all the nasty stuff you see. Then, use cotton balls soaked in alcohol to scrub the bases. For really grimy spots, try a pencil eraser.

- Take another alcohol-soaked cotton ball and clean the rest of your keyboard, the raised parts that always show.

- Once again with the alcohol and cotton, clean all sides of each key before replacing it. This is when you’ll need that photo.

I’ve given you the heavy-duty method, for a really filthy job. You can now keep everything clean, for a long time, by using compressed air around and under the keys.

Perhaps you already know this, but before you begin any of the above methods, disconnect your keyboard from your computer.

I just tried removing one of my keys with the keyboard connected, and I think it freaked out my mouse!

Throw it in the recycle bin, get a new one.

Yeah, you can pull the keys off. But, like someone said, take a picture first.

If your keyboard is cheap, as in it came with your computer or paid very little for it you don’t want to pry the keys off it.

Simply:

1: Unplug it
2: Flip it over
3: Undo the screws on the bottom (they will vary in lenght so keep track)
4: It should easily come apart.
5: The keys are most likely attached to the top part of the housing.
6: Wash the top part in a bin of soapy water (gently swish it around)
7: Repeat the above with a few rounds of clean water
8: Put it in a hot room while you sleep to help with evaporation
9: Dust the bottom part with compressed air
10: Once you are sure the top with the keys is totally dry reassemble.

This is how I do it. But keyboards are simple and I wouldn’t worry about taking it apart.

@Brian1946 and I think it freaked out my mouse!

I can just imagine your mouse running around chasing its tail!