You will get better and better at this with practice! I had this problem too, my acknowledgment of my gut instincts was not quick enough. Also, I far from always knew what my 'gut instincts' actually were (as I'd been quashing them for so long)... And sometimes I'd be muddled... For example, when I felt I didn't want to do something, was that because I was purely fearful and whatever it was would actually be good for me and I'd find it enjoyable/beneficial, or was wanting not to do something what I actually needed and wanted and was best for me?... and so on and so on...
So, what I did and still do, especially when I'm taken off guard, is to buy some time and say "I'll think about it" just like
@Cactusflower does:
Then if I decide I want to do whatever it is, the door may be still open after I have thought about it (if the door gets closed, then it's just too bad and I accept it). If the other person comes back and says 'have you thought about it?' and I've decided 'no' then I say 'no' and if they want an explanation as to why I'm saying 'no' I just say 'I've thought about it and it just doesn't suit me'. If they keep pressing for an explanation, then I just keep repeating 'I've thought about it and it just doesn't suit me' and move the conversation away from the issue. It stops the manipulation by those who wish to manipulate you.