You're great, really, "It's not you, It's me". Cut the BS people, and come up with something a little more original. Or just be honest, either way. These sayings will probably come as no surprise to you, whether you've been on the giving or receiving end of the conversation.

1.  I don't deserve you . . . you're too good for me.

2.  I need space.

3.  You should be with someone who can treat you the way you deserve.

4.  I think we'd be better as friends.

5.  I'm not ready for something like this right now.

6.  And we couldn't leave out, the ever ambiguous:  It's not you . . . it's me.

So why do these cliche's stay alive despite their tackiness? I think Neil says it best:

http://www.femalefirst.co.uk/relationships/break-up-325171.html