We all know how it feels. Here's what I've learned EDIT: Applies to both genders (and all orientations derp)

1. Someone has been through this before you - so you can do it too. Believe in yourself.
2. Before your ex, there were friends. Go see them. You must have at least one friend who she doesn't know, and who you'd like to catch up with. You’re going to hang out with him for a while now, and then another dude, and then another. 3 is a good number, so your group of 4 friends should build a real sausage-community now, go to dinners, movies and other fun stuff together.
3. Instead of looking for girls to date, just try to enjoy being single. I know that's not what a lot of you want to hear, but it's important that you are comfortable being around yourself. Look, the sun shines whether you’re single or not. If you don’t get the wisdom of this, then sit on a fucking chair on your porch and stare at the sun till you get it. Eventually you will. Also, if the breakup was really bad, then you need to sort some shit out. Chances are this affected your confidence. Take some time in the sun to reflect on what the bad times were. Then, remember the good times. Grasp the best memory those days, and make that your label for why you don’t regret being with her. She may have been a cheater, or she may have broken your heart – but she still meant something to you. Find out what it was, and leave it at that. You can reminisce when you’re old.
4. Chances are that you're male if you're reading this, so hit the gym. It doesn't matter what your body type is, scrawny, average or buff dude. Go now, run some cardio for 25-30 minutes, and lift weights for 20 minutes. If it's the first time you've lifted weights, then it will be a life-changing experience. If it's the first time you've lifted weights since you dated her, you'll notice your sex drive will start to pick up again. Most importantly: Take breaks, one day off between every session works for me. (If you don’t have an available gym, you should get the following: One big rugsack for hiking, fill it with firewood or a large tank of water. This will be your weight. Now go for a 45 minute hike, and add/remove weight to build stamina. Repeat.)
5. Now you can go look at women. On the street, the bar, the mall, whatever. But don't go dating them. Just have fun with your friends. This is what your focus should be on. The exception to this rule: If a girl offers you a drink, take it. Besides that, no hunting. Fap if you must to get the stress out, but don't do it more than once a day. Try this for a month and see yourself turn around. You need to appreciate physical and intellectual beauty. Mind and body.
6. Since this is a list, I recommend that you buy a dayplanner, or make a schedule for your month. Look up the cinemas in your area. What awesome movies are coming out this month? Mark them on your calendar. Likewise for games. If you're a gamer, there's lots of great movies coming out this month and the next. Diablo 3, Guild Wars 2, and then there's the other great stuff, like Skyrim, Mass Effect 3, etc. You have time for this now. Have fun.

Check FB for birthdays, parties, etc. And go to them. Avoid your ex. If she sees you, greet her politely but curtly, and walk over to a new group. Talk to them instead. Make new friends. You’re living your life now, and you are priority 1 now.

This advice isn't meant to prevent you from becoming a loner. It's meant for you to remember that you lived a full life before you met That Girl, and that you were an awesome guy before you met her. Life is for living, now go live it. If you ever feel shitty and want advice on cool stuff to do, come back to Reddit. The sun shines no matter what. Go get some sun.