It's pretty simple, but not easy.

The person breaking it off feels bad about no longer wanting to be in the relationship so this is how they think they are making it easier on you. Whether you know it or not, I think they actually ARE making it easier on you. Bear in mind that if you really like this person it's not going to be easy though, no matter what. There is no solution to your pain but time. And time will take care of it. But hearing "It's not you it's me" or some other thing that you know is just bullshit is a lot better than if they said "Well, my preferences have changed and I think you're ugly now" or "Well, your cock is too small" And when they say "we can still be friends" -- that's obviously just stupid, but you're best off to just say "yeah of course" even though you know that in reality you will just fade from each other's lives. There is no need to state the "truth" to each other b/c really, you both already know it. I know how it can bug you though, I've surely been in your position. Today I feel very confident in any "dating" situation no matter what happens, and I have this confidence b/c I have experience from being in so many situations exactly as you've described. Just try to take it easy and move on to someone else, this won't be the last time this happens to you, but you'll handle it better each time it does happen.
To answer your question, some of the stupid ass things I've heard:
"I have to focus on my studying so I can become a doctor" -- this particular bitch said this as a Junior in HS and currently she lives with her husband in his parents house, with a baby.
"My life is just so busy right now! I wish we could have met like, a few months from now, then it would have worked out great! Be sure to call me b/c we're still friends!" -- I told her the ball was in her field and she could call me. She agreed that was logical. I never heard from her again. Hahaha
"I was in the shower, didn't hear you call" -- apparantly this girl showers all 16 hours of the day she isn't asleep.
"No." -- This one sucks, but at least they're honest!