Generally I have no real problem saying 'no' because I'm a terrible bitch; but hey, just in case....

Gotta work!
When I really really really don't want to do something but will look like a twat if I say no.

Nein!
Just start talking in another language.. works every time!

Got my period... soz!
It pays off being a woman sometimes. Especially if you say this to a man...

.. they just back away, very slowly, with their arms up as a shield.