Unadulterated luck. Three of my main characters have been in existence for some 12 years or more. A lot of them started out in role-playing games, and developed out from there. Usually, I start with trying to work out what I want from the story itself, gathering the basic outline and deciding genre. From there, it's to the characters that I need to achieve the basic goal of the story: noble warrior, sneak thief, hacker, accountant, etc. When the basic concept is worked up, I go into more detail to figure out what I need from the character, both in personality and skills. Sometimes I get lucky, and background and motivation for the character comes easy. Sometimes I have to fight them all the way, and keep reworking them until they fit the template that I need for the story. It always seems to come easier when I'm working with someone else. Brainstorming has led to some fantastic collaborative works.