Step 1: Accept that it's done. Don't hang on to "maybe we'll make it work someday", don't worry about what could have been done to change it.
Step 2: Cry, write sad poetry, listen to sad angsty songs.
Step 3: Get angry, it's okay to get angry and let it out.
Step 4: Remember the good times and the bad. Be able to do it without completly sobbing your eyes out.
Step 5: Be able to envision them with someone else. Get used to the idea and be able (if possible) to be happy for them.
Step 6: Date someone else
1. ZERO COMMUNICATION! UNDER ANY CIRCUMSTANCE!
2. Keep busy at all times. Prevents spontanious crying sprees.
3. Refrain from facebook/twitter/tumblr/booksie/social network stalking. Watching them pretend to be happy will only depress you.
4. Do not send any of the sappy poetry or stories from step 2 to your ex (an extention of rule 1 really).
5. With the exception of step two, which should not be allowed to last longer than (for me) a few days, no wallowing in pity or general sadness. The longer you let it drag the longer it will take to heal.
6. Guys are cool but don't let the loneliness make you jump into a relationship you aren't ready for. Rebounding people sucks all the way around.