You haven't spoken to your best friend in months. Maybe you got a new boyfriend she didn't like, or maybe you found separate hobbies. If your heart feels chopped in half by the split, you may want to win your best friend back. However, not all friendships are meant to last forever. If you think yours truly is, prepare yourself to take a plunge as your old friend may or may not feel the same way.
Be objective. If you examine your former friendship as objectively as possible, you'll be able to pinpoint the real reason why you want to win your best friend back. If you have been best friends since grammar school, but the friendship eventually became a convenient arrangement (for example, if you shared rides to work or helped each other with homework), you may not really want the friendship back. Sometimes people just grow apart, and that's OK. However, if you believe that you and your best friend were truly "best friends forever," you should do your part to try to win your best friend back.
Figure out the cause of the breakup. Whether you grew apart or had a big falling out, be willing to accept any blame that might be yours instead of pointing your finger at your former best friend. Hopefully, your former friend will be able to take a step forward as well. Your breakup was probably a good idea for awhile, but if you have both since matured enough to put your differences aside, you can truly embrace what you have in common.
Take the first step forward. If you really want to win your former best friend back, you need to be humble enough to say "I'm sorry" without a guarantee of reciprocation. Make a casual call, send an e-mail or write a letter like you used to in high school, but avoid overwhelming the person with profuse apologies. A simple "just thinking of you" will do in the beginning. Allow time for a response. In the meantime, silently forgive any perceived transgressions by the other party without making an argument over who did what to hurt whom.
Wait for the other party to respond to your message. If you think you've been ignored, offer some extra time to think it over. Realize that there may be hurt feelings involved, and that the other party may be going through work stresses or other emotional upsets at the moment. If you get no response within a few weeks, try again. If that yields nothing, you may need to move on.
Embrace change. Whatever the response, don't expect your friendship to go back to the way it was before it ended. If you get a response, laugh about shared memories and catch up on new stories over coffee. If you get no response, get involved in new hobbies with people with whom you have shared interests. You may find that a new best friend with your same sense of humor is right around the corner.