Start with "Yeah"
Think about it. "Heather, do you want to help me reorganize my garage?" "Yeah...no."

The "yeah" kinda lightens the fact that the answer is absolutly a NO.

The passive aggressive approch
This is great when you are asked to do something, attend something or go somewhere that someone with the best of intentions wants you to go and they really me no harm and think it would be fun but instead of saying no and hurting their feelings, you would much rather dig out your own eyeballs with rusty spoons. Phrases like, "That might be fun." or "I don't know what we are doing that weekend." or simply "Uh-huh." are noncommittal and easy to blow off later. I realize I am going to hell by the way.

Just start crying.
Espcially if it is a man asking you to do something. They will have no idea what to do, assume you are on your period and just throw a tissue at you and walk away as fast as they possibly can.