Lol...well first things first...there is no way to break it off with someone, without pain, if the other person cared for you. Having to let go of anything you care about hurts, whether it is a paper cut or a deep gash, pain is pain.
I think the ONLY way to break it off with someone is to sit down and have a chat. I think that the other person deserves some sort of explanation for two reasons:
- Closure
- They invested time, feelings, etc into something they felt promising. They need to know why it needs to end.
And yes, that whole "its not you its me" bit sucks, but sometimes that is the case. Just let them know exactly what it is about you that cant be involved with them. lol. The main thing I find that sucks the most about a break up is you cant control how the other will take the news. That part is based totally on the type of person they are, and how they handle stressful situations.
I hope that this break-up goes smoothly for you

Be honest and you can leave with a clean mind