say no immediately
I ought to adopt Sean's strategy described below Sivers' If you’re not feeling “hell yeah!” then say no:
Emperor Napoleon said he loved how his wife would always say "no" right away, to give herself time to think.
I do this trick too, both out loud and inside, with others and with myself. Everyone would hate me if I kept them in hopeful suspense as I was thinking in their presence, but no one minds if my quick "no", after I figuratively or literally "go away and think" then turns into a yes.