There are a few ways you could cleanse your crystals. I will go through a few of them and the pros and cons of them.
First you could take your stones to a stream, river, any sort of running natural water source. Let the water run over them while visualizing the negative energies leaving them, and them being clean and ready for re charging.
Second you can use incense smoke to cleanse. You could use this for crystals, stones, tarot cards, and pretty much any magic item you wish to cleanse. Use the same method as above for visualization of cleansing.
Third you could use regular tap water with some rock salt added to it to cleanse them. Beware the salt could potentially fade the crystal some say, I personally have had no trouble with with. Same visualization as above.