Use visualization and deep breathing. This is more of an in-the-moment tip. When you're gearing up to talk to that cute boy or to give that speech, that's when you'll need to use these little tricks.
Visualize yourself doing awesome and it all going swimmingly. When you expect success, you might just get it. But when you expect to fail, you will.
Take a deep breath. When we breathe too quickly, our heart rates go up and our brains start thinking "fight or flight." You're just making yourself more nervous. Since you won't be attacking any wooly mammoths anytime soon, there's really no use for that.