Remove yourself from the scene for a while. Stay away from group activities where you might see your former boyfriend. Ask your friends to socialize at a different spot for a bit, while you regroup and remind them that you don't need to know if your ex has been spotted with a new love interest.
Delete her from your social contact list. Taking her number out of your cell phone and "defriending" her on Facebook takes away the urge to call or surf her profile page to torture yourself with any new romances she may be brewing. You will be thinking about her enough in the beginning; try to remove any additional cues and you will get over her much faster.
Indulge yourself. Take a weekend ski trip, hit the spa, or travel out of town for a concert or sporting event. Bring along a supportive friend and try to reduce the amount of time you spend talking about your ex.
Volunteer somewhere. Doing something for someone else gets us out of our own heads and away from the self-pity of a breakup. While there, put your best foot forward and try not to think about how much better your time would be if your former girlfriend were with you.
Talk to a therapist, even if it's just for a few sessions. Chatting with someone not intimately involved with this partnership allows you to be truly honest about the romance. You may discover things about the relationship that will make it easier to move on.
Give yourself time. Removing romantic feelings from someone is painful and difficult and does not happen overnight. The kinder you are to yourself and the less time you spend wishing things were different, the sooner you will go to bed one night realizing you didn't think about your former love once that day.