Thanks for the story. You reminded me about something Daniel Pink said in one of his pinkasts. If it's not a hell yes, than it's a no. Giving yourself time to say yes first in your mind and check if it's a real yes sounds like a great strategy.
We should teach assertiveness in school because is indeed import not only to say No but the way you say it. You can be polite but ferm or you can be rude and this is not anymore about boundaries.