There are lots of ways to keep your rock clean and tidy, but this is one of the best methods.

[edit] Steps

First, get something like a bowl, or sink to bathe your rock.
Get a bottle of shampoo and put on a small dot of shampoo on only one side. Then rub, and repeat on the other side.
Rinse!! To really get that sparkle on your rock, put on an extra blob of shampoo and scrub the rock really hard in your hands, then rinse. Scrub like you were actually washing your hands.
Then wrap up your rock in either a napkin or a small towel, it doesn't matter.
Congrats, you now have a healthier cleaner rock!

[edit] Tips

Rough skinned rocks will need more cleaning and attention to cleaning because there will be more cracks and crevices for dirt to get into.
Make sure you have warm water in the bowl!