So this is for slow burn game (college game, social circle game, etc) where you know the girl and seeing her again is inevitable.
me: hey [HB] lets go grab a bite tmo. i'm free after 5, you good?
hb: hey sorry i'm going to the gym with a friend
me: that's cool, but i better be able to feel a six pack next time i see you
So next time I see her, gonna poke her in the stomach (or rub) and joke about it. Keep it light-hearted, and keep the Kino up.
There, that's one way to handle flake.