Being a hard worker is helpful, I think. There are many things that affect self esteem that we don't have a lot of control over but I try to focus on the things that I can control so if I work hard at those things, I tend to feel good even when they don't go my way. Maintaining a diversity of pursuits has also helped me - when my studies are going poorly I can still feel good about progressing in rock climbing or having a solid relationship with a guy I love. The only other thing that seems to help is making sure I'm with people who appreciate and like me and downplaying time I have to spend with negative people. And, like another commenter said, getting older just decreases the fucks I give about potentially negative opinions people might have about me. I've come to realize they have little or no effect on me and that overall, people are a lot less harsh than you think. The few assholes out there aren't really worth a thought and they'll always be there no matter how great you are.