I like Jared's rule of 3 things.  For me, in general, it breaks down like this:

1. Sense

2. Drive

3. Hook

**Sense**: Pick one sense way of describing them...sight is the most common but smell works too and touch is fairly uncommon.

**Drive**: Pick one drive they have, something they want.

**Hook**: One other detail, something you think is cool or something that links them to the adventure at hand (ideally, both but don't get to hung up on it, an NPC can come and go every so often and it is okay).