1. Be Honest. It's not a crime to want out of a relationship, but you don't have to let it drag on, cheat on your partner, or make things worse then they have to be. When you start feeling the love fading, talk about it. If it can't be fixed, at least they will respect you. Maybe they even have been having similar thoughts.
2. Be Gentle. So you think your partner is a complete jerk and sometimes you think that meeting them was the worst thing that ever happened to you. Chances are you don't really feel that way and time will heal the wounds. Chances are you will want to share her company again when the time passes.
3. Do it in person. No one wants to hear the relationship is over the phone or via email. Be brave enough to tell it to their face that you don't want to be their significant other anymore. If you do it any other way, chances are you will never talk to them again. Ever.
4. Make the time for them. If you say you want to be friends, then take the time to actually do things as friends with them. Don't lead them on certainly don't continue to sleep with them, but don't be too busy to make them feel like they are a friend. Go to lunch. Go see a movie. Don't let them think they are losing you all together. It's like you breaking their heart all over again. One more tip: If you find someone new right away, don't talk about your new lover around them too much and certainly don't suggest the three of you hang out at least for a while. That's bound to be an uncomfortable relationship for everyone.
5. Don't Use Cliches. Don't use the tired old lines to break up. Tell them what is really going on. No one wants to hear it's over, but "It hurts me that you don't support my dreams" is a lot easier to swallow than "It's not you, it's me!'