A rushed example by Roland King and Emile Christ
Despite being rushed, it tells quite a lot. All the expressions are different and unique in their own way. They're never the same used twice, as John K so famously used in his cartoons, and you can identify them by even the lamest angle.
If you know how to draw, then expressions for a character show be unique, different, and you can't use them again. I can post up some production art later.
You can use all kinds of inspirations, for this is okay. Taking examples from magazines, video games, cartoons, animated short films, anything. There's a hell of a lot of programs to help you, such as Photoshop and Paint Shop Pro. If you know what you're doing, be patient and TAKE YOUR TIME. Roland and Emile's picture, although showing some great expressions, is supposed to send a message: DON'T RUSH. If you can take the time to think up unique expressions without the final stage looking weird, rushed, or unfinished, then you're on a roll. You just need the direction, and the PRACTICE.
Expressions can help show a character's personality as well. You'll always have to be thinking, but take the time and it'll be fine. If you want an angry, irritable character, draw angry and irritable. If you want stupid and unknowing, draw stupid and unknowing. Happy and joyous, draw happy and joyous. I highly recommend buying a couple of art books, to go through step by step, or maybe buy a cartoon DVD if it has an art gallery. Fan art is also a good technique and example. It all matters.
As I said before, I'll post up some art maybe later or sometime tommorow. I would if I wasn't in a library right now (heh, heh)