i think where im concerned its having something looming over me that causes the problem. so what ive been trying to do, when i feel good and think i can do something, is say:
"that sounds great! id love to.. but im not sure how ill be feeling when friday rolls around, if you dont need to know for sure right now then lets assume its on, but give me a ring on thursday afternoon to be sure."
no one has ever had a problem with this approach and it beats me festering and getting anxious as the time approaches knowing people are counting on me to be somewhere. i also tend to bow out less because i dont have the added stress to perform well. when i do back out the other person has known this was an option all along and isnt disapointed.