I agree that it's not about being hard or a bitch. It's really about confidence and calmness and knowing what you won't put up with, but in a calm way.
I put up with decades of shit from my mum before I managed it, because I was just so scared of upsetting her and the fallout - although she was happy to upset me all the time. I started pulling her up on things and calmly saying things like "that's incredibly rude, you must know that's not an OK way to speak to people" and "that's clearly not true so I'm not going to respond to it" and walking away or changing the subject. Or even just simply "I'm not bothered what you think of my weight/jumper/paint colour choice - I'm happy with it" in response to her endless criticism and sneering.
She of course hated it and made out that I was a hard bitch, for simply stating the facts and not putting up with abuse. But she has reined it in a lot, because she no longer gets the drama and power play she craves from treating me like that. It's never going to be a close relationship, but actually it wasn't before - she just deluded herself that it was.
With less personal situations, like the school mum who's always trying to palm off her child on me, I have found the phrases "No I'm not available for that" and "No it's not possible then" are good - you're not saying why, so they can't argue. Look them in the eye and don't be embarrassed or apologetic - they're not are they?
Today a man who hangs around our local cafe tried to get me to give him my stamped loyalty card. I don't know if he's homeless / has MH issues or similar, and I would consider this in some circumstances, but he was quite pushy and made me uncomfortable. I just said "No this is mine, I want to keep it." It's very freeing as PPs have said to just say no, and stop apologising.