Cleansing crystals regularly is necessary as they will absorb negative energy. The following are my recommendations for cleansing your crystals:
WATER: A nice cool water bath works very nicely on most crystals. It is quick, easy and you can do it any time. Simply run it under running water and visualize the water cleaning out any negative energy. This will get them clean as well as cleanse their energies. However, some crystals you need to avoid putting in water. These include, but are not limited to: Malachite, Selenite, Turquoise, Halite.
MOONLIGHT: I have not yet met a crystal that does not enjoy the moon light. The best time is during a full moon. It's ok if you miss the actual day of a full moon. You can catch those moonlight rays on the day before, day of, and day after a full moon. It is ok if it is cloudy or raining. Your crystals will still get the lunar energy. However, if it is going to rain make sure that none of your crystals should avoid water.
SAGE AND INCENSE: Sage is another wonderful energy cleanser. You simply run your crystals through the smoke. It also cleanses any of the areas around your house. If you do not have a stick of sage available incense work also.
USING OTHER CRYSTALS: Not everyone has the ability but I thought I would mention it since it is one of them I use. Your crystals can be placed on top of an Amethyst cluster, put in a bag with a tumbled Carnelian, or surrounded by Clear Quartz Points. I personally use an Amethyst cluster when there is not a full moon handy. Keep in mind these crystals mentioned above will need to be cleansed themselves. But it is so nice that they can help out the others we work with.
SUNLIGHT: Many crystals love to sun bath, especially ones that are red, orange or yellow (like Carnelian and Sunstone). You can put them out in the sun for a few hours. There are some crystals that will fade in sunlight or change color. These include, but are not limited to: Amethyst, Ametrine, Citrine, Fluorite, Rose Quartz, Smoky Quartz.