Being popular isn't about being mean or arrogant or putting other people down. It's about finding people who like you for who you are. Be yourself as much as you possibly can, and try, I know it's hard, but try not to worry so much about what other people think is cool.
Try not to set perfection as your goal. This is something that I always struggled with, trying to be like a perfect person. Perfect people simply don't exist, and if you set perfection as your goal, you're only setting yourself up to be disappointed.
It's really important, also, to be super confident. It almost goes without saying, but confidence is key. Not only do you want to be OK with yourself, you want to be OK so that when your friend comes to you and says, "You know, that thing you did the other day really bothered me," or something like that, you want to be able to take that and not be hurt by it.
They're not saying it to hurt you, they're saying it because they want to improve the friendship. So let it, let it strengthen the friendship instead of taking it as a putdown.
Be nice. It might seem counterintuitive since so much TV and so many movies, all the popular girls are really mean, right? It's OK to be a nice girl with a lot of friends. That makes you popular.
Just remember, if you don't feel popular one day, it doesn't mean that you're not popular. Popularity comes in and out, and it's the friends that you make that really mean something to you that last a lifetime.