One of the keys is, and it may sound funny, talking about characters with super powers, but one of the keys is to make your characters as realistic and believable as possible. Even if they have super powers, you say to yourself, "Well, if somebody had a super power like this, what would his life be like? Wouldn't he still maybe have to go to the dentist or wouldn't he have to worry about making a living? What about his love life?" You've got to make characters that your reader can believe exists or might exist.