After getting out of a terrible relationship with a NVM, I had a really hard time setting boundaries and actually sticking to them. I finally found something that's working for me so far, so I'm putting it out here in the hopes it'll help someone else.

Write them down. Write down every boundary you can think of, because it is a hell of a lot harder to say a man respects them when they're written down.

For example, let's say you refuse to have sex with someone new for 3 months. But inevitably, there's just one guy who seems perfect for you. You have amazing chemistry, he's kind, thoughtful, generous. The only thing is, he's pressuring you to have sex. He started withdrawing once you told him you want to wait, and now he's giving you the cold shoulder. You and this guy get along great, you think, and you don't want to lose him. What's the harm in having sex a little early? So now you think that two months is the perfect time to wait. Maybe he leaves you for an easier lay, and you decide to start only waiting a month. That's plenty of time to vet, right?

Or maybe he makes a kind of sexist joke you thought you wouldn't allow, but it's "harmless". Still, you call him out on it, and he's mad you're a "buzzkill". So now you only have a problem with rape jokes specifically, or something blatantly sexist.

Just like that, your "ironclad" boundary has been cut down. The "right" (read as "most manipulative") guy can bend, and eventually break your boundaries. Don't let him. Boundaries in your head are a lot easier to warp during a date than ones on paper. You can tell yourself that the sexist joke is fine, or that he's not pressuring you that hard to have sex, but when you get home and see it in black and white (or purple because fuck yeah use the fancy pens you deserve it), you have to accept he isn't respecting your boundaries. And once you realize that? GHOST HIM