Get the logistical things out of the way. If you and your ex have left various possessions around each others homes such as clothes, books, and CDs, arrange to trade them back in a neutral location. Resist the urge to prolong this interaction.
Accept the relationship is over. Don't spend time wondering where it went wrong, or if you could have done something to prevent it.
Talk to your friends. While friends can initially provide a shoulder to cry on, don't allow this to go on for too long. Instead, go out with friends and do new things that will take your mind off the breakup and your ex. If your friends are constantly saying how bad they feel for you, remind them you are ready to move on.
Don't bottle up your emotions. Find a good friend or close family member you can talk openly with, or even consider seeing a therapist. You can also write, do art, or exercise as a way of releasing pent-up negative energy.
Sever all ties with your ex. Doing this will remove the temptation to rehash the breakup over and over again, and will also prevent you from having an ill-advised rebound "fling" which will only serve to confuse both of you. Remove your ex from your social networking sites like Facebook or AIM, so you will not be reminded of them or be forced to see when or if they begin dating someone new. Depending on how the relationship ended, you may be able to be friends eventually, but it is important for both of you to have space immediately after the breakup.
Find a new hobby. Take up a new sport or musical instrument, or try cooking something you've never made before. This will focus your energy on something positive, and you may find something you're really good at.
Get a makeover. Get a haircut or a manicure, or even just treat yourself to some new clothes. This is a simple way to do something for yourself that will make you feel better. It will also create new associations outside of your memories of your ex.
Rediscover your social life. Go out with your single friends and have a night on the town. Remember all the good things about being single.