The ones you keep in tanks? If they get stained or something? If so, I suppose the safest way is to scrape them and rinse them in tap water until they're clean - no chemicals! It would be even better to clean them in tank water.
A good strong brush usually works. I have in the past used bleach and then soaked them over night in heavily dechlorinated water.
What are you trying to clean off of them? If they have ever been used for plants that might have been fertilized or had other chemicals in them you can't use them in your fish tanks.