Summarize:

Take down all of your posters with pictures of them, and don't see the new movie they are in or listen to their new song. Don't talk about them with your friends or family, and don't spend time on the internet researching their lives. Everyone deserves privacy, even famous people. You can do these things gradually over the course of several days, or stop it all in one go, like ripping off a Band-Aid. Whichever way you choose to do it, make sure all of the things that remind you of them eventually make it out of your life! Try not to be jealous if you find out they're dating someone else, or happy if they're breaking up with their partner. It doesn't bring you any closer to them, and it only makes it harder to separate from them. Concrete goals that focus on the bigger picture, like how you can improve yourself or do something nice for someone else, will help you far more than dwelling on an unlikely possibility. Make sure your goals are reasonable and attainable. Think about how you want to study hard and get an "A" on your next paper or test. If you're working on a new move in karate class, imagine yourself being able to do it perfectly, and practice when you get home. Help someone in need by volunteering at an organization, or doing something small like giving up your seat on the bus for an elderly person. If you are having intense thoughts about this celebrity, an engaging but calming activity can help get your mind off of it. Finish that poem, book, painting, or other project you've been working on. Go for a walk and look around at all the beautiful trees, houses, and people that are right in front of you. Grab a coloring book or a print-out of a detailed pattern like a mandala and some colored pencils. Color to your heart's content, and try to stay inside the lines. Get some exercise by asking your friends if they want to play a pickup game of soccer or frisbee, or go on a run by yourself.
Remove things from your life that remind you of your crush. Replace daydreams about them with thoughts about goals you have for your own life. Do something else that you like to do.