My favorite piece of advice received currently is to keep a self-esteem file. Write down every little compliment you get, no matter what it is--I once wrote down that my bf said "woohoo!" to me. I blush, but it made me feel good. Write down every little thing you like about yourself. (I put down stuff like, I like that I'm loyal.) Keep the file where you can look at it every day, and write down everything. It actually does help. Keep doing it until...well, frankly, I think forever. It can't make you less healthy.
Also, I'd highly recommend that you take a look at some information on improving self-esteem. There are a lot of websites on the internet and self-help books at the library for free.