There are actually two types of "cleaning" -- Clearing and Cleansing. Clearing your deck is appropriate to do between readings. Cleansing is a more involved ritual which should only be done if warranted.
Clearing your deck is simple and quick. Here a few ways you can do it:
1) Place your cards face-down in a stack. Hold your hands several inches above the deck, palms down and crossed at the wrists. Form the intent of clearing your deck of any residual energy then separate your hands with a quick, forceful motion.
2) Hold the deck in your dominant hand and sharply rap one side of the deck 3 times on the table.
3) Lift the deck to your mouth and blow on it.
Cleansing your deck is more complex. You may wish to cleanse your deck if you have bought it used, if it has been damaged or if you feel it has been exposed to negative energy, or if you feel that it has gotten "tired" and could use some re-charging.
The main ingredient in any cleansing ritual (besides your deck) is a "purifier." Here are a few examples with suggestions for their use:
Salt (natural sea salt is best) - After wrapping your cards in silk or cotton, sprinkle salt on top of and around the deck. Leave the salt there overnight then sweep it up and flush it down the toilet.
Sage - In a well-ventilated area, light a bundle of sage and pass your cards through the smoke until you're comfortable. Repeat if desired.
Candles - Place your deck within a circle of white votive candles or tea lights. Leave it there until the candles have burned down. (Do not leave burning candles unattended.)
Incense - Pass your deck through the smoke of sandalwood or frankensence and myrrh. If using stick incense, wave the smoke over and around the cards in a circular motion.
Crystal or a talisman of your faith - Place the talisman or crystal on top of your unwrapped deck and leave it on your altar or on your windowsill in the light of a full moon. When you've finished, hold the crystal or talisman under warm running water to clean it.
If you're looking for a serious cleansing ritual with lots of ingredients and incantations, you'll find one Here