When my keyboard keys get dirty, I moisten a quarter of a paper towel with some Fantastic (moist only, not wet!!) Then gently rub each key one at a time to remove the built-up dirt. From time to time I refold the paper towel to use a clean surface. Rub just the tops of the keycaps in a circular motion. Be careful not to let any liquid run down the sides of the keys. (Even a moist paper towel has some liquid on it.) Run a dry towel over the keyboard to pick up any remaining moisture.
Before cleaning, either disconnect the keyboard from the computer or shut down your computer/
Also, do not spray the keyboard with anything or use any powdered cleaners.