Don't start your character off with a name and a physical description.
How you start the characterization process shouldn't matter, but it does. If you start a character off with a physical description, you'll be tempted to look no deeper than that. Say you start with Mark Flimimaro, 55, well-built with brown hair, shocking blue eyes, and a habit of biting his nails when puzzled. When Mark here gets puzzled by the bad guy, you're going to fall back on the nail-biting, and by the end of the story he's not going to have any fingers left.
What makes your character isn't what you look like or what you do when you're nervous, but what you do when no one is looking. Will you give back the fifty that man just dropped? Will you stay faithful to your mate when a chance comes along for a no-strings one-night-stand comes along? When the pressures on and you can do what's right or what's easy, what do you do? The answers to those questions make you human, and they'll make your character human, too.
The second reason for not starting off a character with a name is emotional baggage. Not the character's, but yours. Humans generally form memories associatively. If you name your protagonist Barry and every Barry you've met was a jerk in disguise, you'll project that onto your protagonist without even realizing it. That projection will influence your story, maybe in a good way, maybe in a bad way. In any case, it's not worth the risk. The above also goes for physical characteristics. Say a redheaded nurse was especially kind and considerate towards you when you were in the hospital. If you go and make your villain a redhead, you may have problems down the line making that redhead do something nastily villainous.
It's really best if name, appearance and quirks are put in later. For example, when you need the character to do something that tips Dumbledore off to a plot to kill Harry.
Don't overdo the accents
Accents are fun, and can really make a character stand out. But don't drown in it. A suggestion of an accent is all that's really needed. Writing, "Howz ayre y'all doin' toodee?" makes you look like a moron. "How are y'all doin' today?" is enough to make your point. For some really good examples of accent work, check Hagrid in the books, and Tria in Lorry's exercises.
Don't rely on crutches
Characters with nervous tics are about at irritating as nails on a chalkboard. If your character does something unique to him/her, like jangling keys when nervous, let it be a minor thing. Mention it now and then for mood, but don't let the crutch take center stage.
Don't tell, show
This one is very important, for two reasons. One: telling can get confusing. If you say your character is scrupulously honest, yet that same character tells a lie four pages later, your reader will notice and either a) get confused, b) think the character is schizophrenic/messed up/dishonest, or c) think you're a moron who doesn't know his/her own characters. What you say has to match what your character does. So don't make a whole lot of definitive statements. The exception to this is first-person dialogue. We lie to ourselves all the time, so having someone con himself/herself into thinking they're honest is believable and human.
Reason two is that telling is boring for the reader, showing is fun. Good books, like Rowling’s, show us what the character feels instead of saying it. Rowling doesn't really tell us Ron is jealous of his brothers and Harry, but shows it. In the fourth book Ron comments, "Why is everything I own rubbish." In the first book his dream is to surpass his brothers. We know that Ron has some jealousy issues before Hermione ever mentions it. The exception to this is a character sketch. You know, those interesting little ficlets where the character tells you what they're feeling. It's okay to tell and not show during those.
Don't sympathize with the character
Sympathy is the affinity you share for the character that creates change, allowing the character to affect you. This is bad. You can never allow your affinity for your character to tempt you into making things easier for them or alleviating their suffering. The hardship makes the story, you take that away and your character feels good but your reader is bored stiff.
If you don't do all the above, chances are your character will have a fighting chance at life. It may even get so alive it won't listen to you, and take the story off on its own path.