One of the most important aspects when working with crystals is cleansing. Crystals are akin to empty containers that over time fill up with energy when used at which point they can become duller and their energetic properties dampened. At this point a crystal should be cleansed and emptied of stagnant energies. So how do you cleanse your crystals? There are a number of ways to do this.
1: My most often used method and very effective way is to put my crystals on the ground outside in my garden overnight. This should be on the soil or grass no need to bury them! So pick a nice spot in your garden and put them there in the evening and return in the morning to pick them up. They should feel fresh and lighter and you can even notice colour changes if they darkened from use.
2: If you have any sage then you can use this on your crystals to cleanse them, make sure to give them a good saging and they will be much clearer afterwards.
3: On a windowsill during a fullmoon, this is the most well known method by far. This is very effective but at times impractical as if you need to cleanse crystals and it is not near a full moon.
4: Out in the sun. So either put your crystals outside in the exposed sunshine or on a sunny windowsill. This also works very well and can charge your crystals too. One thing with this method to be aware of is that some crystals don't like sunlight and will loose their colour quickly. So know which of your crystals are affected by this, some examples are rose quartz, amethyst and opaque calcite types.
5: Running water. A minute or two under running water also works as the water moves energy off crystals. Of course the purer the water the better. Again like putting crystals out in the sun be aware that some crystals do not like water so know which of your crystals are affected like this before you use this method.
6: Meditation and intention setting. If you have a meditation practice you can have your crystals in front of you and put your focus and attention into cleansing them which works really well if you have a strong focus.
There are more methods of course but these cover the main ones. It is really a matter of finding which works best for you. The more you are tuned in with your crystals the more you can feel the difference before and after a cleanse. One more thing I would say is that when you buy a new crystal whether its in jewelry form or a loose crystal clear it before you start using it as the crystals have been transported and picked up by people therefor absorbing energies. This also lets you forge a better connection with your new crystal as the energy is nice and clear.
Another important question is, How often should you cleanse your crystal? Depends on how much you use them really, if you use particular crystals often at least every two weeks. Generally if I use them a bit I do it around once a month. As mentioned when you first get one cleanse it before you use it. The more time you work with crystals the easier it is to tell if they need a cleanse, they tend to feel really heavy and dense or might not feel that great to hold anymore. If you use them for especially deep healing work I would cleanse them after such a session. In the end the more you tune in with them the more you will be able to tell when they need a cleanse.