I'm just like you, and always look kinda messed up or just not myself in pictures. I've tried all of the things sexcichick mentioned, so I guess I'm just not a very photogenic person. Look at it this way though, wouldn't you rather be someone that looks great in real life, but not so good in pictures? I think that's a lot better than being an ok looking person in real life, but great in pictures.
hmm..try taking pictures from different angles, that's what I do:) and make different poses and when you take a picture have like a signtaure smile so take some tyme(5 minutes tops) and jsut practice smiling in different ways so that you can find out the smile that suits you best( open mouthed smile, closed mouth smile, smile to the side) Don't worry you won't look like a fool while your pracitisng your smiles in the mirror! ;) Oh and get photoshop that way you can change things you don't like about your pictures.
Im the same way, but I found that I look much better in group pictures. Its probably because you're just being you, and having fun so you look more comfortable. Try taking group pictures when you're just hanging with your friends, but you can crop it so its just you in the picture, unless you want you friends in it which is cool sometimes, but not always.
SMIZE- smile with your eyes. Think about positive thoughts that make you happy when you take a pic. that way you won't have that fake smile look, and you'll naturally radiate happiness which translates well into pics