You can rinse out the inside of the tank, and the top cap, with warm water, and then dry it with a paper towel or lint-free cloth. The coil units are disposable; once they start to go bad, there's really not much you can do to "restore" them, you just chuck it in the bin and screw in a new one. As for when, that's all up to you. You'll know it's time to replace the coil when you get a bad/reduced flavor, and/or reduced amount of vapor being produced. Some people clean the tank and put in a new coil every time they switch flavors, but if you use complementary flavors you don't have to do this (for example, when switching from a "blueberry muffin" to a "strawberry cream" flavor, you might not mind if the flavors mingle a bit and you get a sort of "strawberry-blueberry-cream-muffin" flavor for a little while), you can just fill up with the new one and it will "take over" after a tankful or so. Stronger flavors like tobaccos, mints, etc. can "linger" and affect the taste of other liquids for the life of the coil; if you use these kinds of flavors you might want to clean it out and switch coils between flavors (or just get a second device and use one for each flavor -- you're going to want to upgrade eventually, and you should always have a backup device anyway in case your primary one fails.) If the coil still has some life in it, you can store it in a plastic baggie to keep it clean until you want to go back to that flavor again.
