Changing juice can be done a few ways. If the flavors are similar you can just wash the tank with hot water and clean off the head with a lint free towel. It's a lazy approach. Some people might wince at my post but honestly, the old flavor taint lasts all of 5 minutes.
If you like a wide variety of flaves then it's more important to clean the head before using it. I have a rotation with my evods. I just drop the old head in a shotglass of cheap vodka for an overnight soak (sometimes it's best to take a few sips of vodka before dropping the head in). Then I grab a dry head and screw it in. Unless a juice has a really strong flavor, washing a tank out with really hot tap water until the smell is gone takes care of the old flave.