Read voraciously, noting which characters strike you as detailed and original and which seem flat and dull. Take note of the cliched character types that appear over and over again in your reading so you can avoid them. Jot down what you like about the better characters and what techniques the authors use to convey information about them. For instance, do you like them because they defy stereotypes? How? Do their pasts intrigue you? What are their motivations? You may also find it useful to write down the worst aspects of characters who bore you to tears. If you can't stand the mustache-twirling villain who does evil deeds just for the fun of it, you'll be more likely to give strong motivations to your own antagonists.
Determine what role you intend your character to play in the story. This will affect some of her traits and what motivations she'll need. If she is the protagonist of a mystery novel, she'll need to be clever and have a motivation to solve the mystery. If he is the romantic interest, he will need to have traits that would attract the protagonist. If he is the antagonist, he will need motives that will put him in opposition to the protagonist.
Jot down a basic description of the character's personality. For instance, you might write, "She's a manipulative, ruthless CEO who wants to steal the technology." Although you should generally try to avoid stereotypes, it's OK if your characters seem a little trite in the beginning. When you flesh them out, you'll give them the quirks and details that distinguish them.
Brainstorm possible motivations for your character. Because you're getting more specific here, this is the stage where you need to break from stereotypes. Using the example of the ruthless CEO, why does she want to steal the technology? You could say that it's for profit, but profit alone is a motivation that readers have seen before. Give her a different motivation or an unusual reason for wanting profit so badly.
Create motivations for your character's personality traits as well as for his goals. If your character is a party animal, it might be because he's an extrovert who can't unwind without others around, but it might just as easily be because he's a burgeoning alcoholic who looks for excuses to drink socially or because he's desperately lonely and hoping to meet Ms. Right at one of the parties.
Write a possible past for your character that would lead to her having the motivations you brainstormed. A character motivated by religious fervor will have a different past than a character motivated by hunger for fame. What happened in your character's past to make her want her goals so badly? What happened to give her the traits you created and the motivations for those traits? If you find yourself writing something that you don't like, either because it takes you too far from the needs of your story or because it isn't coming alive for you, try a different past.
Flesh out your character with the little details that will make him come to life -- mannerisms, personal tastes, opinions on issues. One way to do this is to conduct a "character interview." Picture yourself asking the character questions, then picture his responses, complete with any pauses, gestures or mannerisms. This will let you experiment with the character's voice and his traits.