Many years ago I knew someone who said 'no' all the time to just about everything........ asked to do a favour, invitations out etc. It didn't take long before the requests dried up.
But then came the moaning to me that others had got together but she 'hadn't been invited'.
I reminded her that was probably because she always said no! 'But I would still like to be asked in case I want to', came the reply.
I've always remembered that, and my stock reply when I don't want to do something has always been:
'Sorry, I'm not able to/I can't, I'd rather not on this occasion.
But thank you for thinking of me'.
I like to think that people will remember the thanks and ask another time, because I don't want to not be asked. I also occasionally do something I'd rather not just to balance things out if I'm aware I've said 'no' to too many things for a while.
That way you set some boundaries which will indicate to others that you don't mind being asked, but you won't always say 'yes'.
It has backfired on me though! I once declined to offer a lift to someone because I was genuinely not intending to go to the same event (it was also out of my way). I wasn't too bothered on the company of the people I knew were attending. But then someone I hadn't seen in ages, and liked, contacted me and asked why I wasn't going. 'Come on, it'll be fun' she said.
And I knew with her there it would be, but I felt too guilty to attend and didn't know how I would have looked the other person in the eye!