you were the one being dumped.
try to ask her if she still wants to be friends though, as you might start a great friendship.
never say any of the classic break up lines like "it's not you, it's me."
say to your partner when breaking up "i think it's probably better that we are just friends" but say it nicely and smile. she probably won't get as upset (obviously she will be) as "i hate you, we're breaking up".
don't let her think you could possibly get back together if you have no intention of doing so.
be honest with yourself and with her for the reason for breaking up.
no matter how nice you 