I wonder if it would help to have some 'stock phrases' to pull out when someone asks you to do something? that way you might not feel so under pressure to give a 'Yes' before considering and you can be calm in your response. Maybe some phrases like:
'I'll think about it and get back to you'
(perhaps if it's a really big commitment you need to think about)'I'll need to check my diary'
(perhaps if you know you don't want to do it - you can always put something in your diary if you don't want to lie)'That sounds good and maybe next time'
(as it sounds and you don't have to do everything offered to you straight away + you can always change your mind)
As long as the phrases sound natural like something you would say you can give yourself breathing space without anyone confronting you and if they do you can always repeat the phrase (maybe slightly changed)
'I will think about it and let you know'
'I'll check my diary and let you know'
'Not this time but let me know next time'
Hopefully this should reinforce your position without having to give any further ground to the other person - Unless they're really stubborn/ignorant that should suffice. If they do persist then you might have to be a little firmer and change topic. Have a practice in the mirror/your head or with a friend if you can to give you some added confidence.
Don't know if any other BAFers might have some good stalling tactics, think maybe some self-esteem books and sites might have some tips on being assertive and good luck
