If your getting back together after a long time you will have a few more obstacles then the average relationship break up because in the time that passed so many things can and probably have happened so be prepared to work a little harder then the average guy here.
Keep in mind this can be done it happens often because sometimes the time a part of exactly what's missing when people try and pursue their ex's too soon.
Figure out why you broke up and what the real reason is behind it all before you start your hunt.
Did you do something wrong or was it their fault? And if it was your fault is your ex over whatever you did or would he or she still be very upset with you.
You need to know where you stand before you go after them or you may get burned really fast, but if it was a mutual break up there shouldn't be that many problems.
Now you need to figure out how long you have been a part be it months or years because depending on the amount of time because his or her head could be in a very different place right now.
Do you know where they are living and are they seeing anyone currently? It might not hurt to contact some friends who knew her as well and get some information.
You don't want to come off as a bit of a stalker like the movie there is something about Mary, but Ben Stiller had the right idea in mind bumping into her.
If all is well you might consider something mild like inviting her out for coffee or a lunch, but I would avoid dinner unless she suggests it because it's a bit to personal in my opinion.
Get some information and go from there, but make sure you don't come on to strong even if he or she happens to be available because it might take your ex some warming up to the idea to give you guys another shot.