1. Give them a name, make it stand out, maybe something funky. Editors like names with personality.
2. Go over the simple stuff; gender, age, grade, skin color, hair, eyes? Any interesting physical traits? Scar or limp? Do they have a special ability? How'd they get it?
3. What does your character like to do? Write down a few things they like to do during their free time.
4. What does your character hate to do? Write them down. Also, write down why they hate them-are they just not good at it or is there a deeper reason.
5. What's their setting? Describe for yourself their home, room, orphanage- whatever.
6. All main characters need a flaw. Think of the seven deadly sins, those are ripe for the picking. Just make sure the character is still likable.
7. What is their deepest desire? This will often clue you into your plot. How far would they go to achieve it? What would happen if they didn't get it? Write it out.
8. Just for fun, throw rocks at your character. Think of situations that would totally throw your character off and stick them in the middle of them. Readers love non perfect characters and like to see them react.
9. Give them a habit or distinctive mannerism.
Are you a visual person? Draw your character or cut out a picture from a magazine and hang it on the wall next to your computer. You can even hang the answers to the questions on the wall.
Good Luck and Happy Writing.