first of all, don't put too much pressure on yourself, it's really tough to hold back sometimes. if you can't hold it back, that's okay! sometimes, people do need to be told when they're being cunts sometimes.
but what does help me a bit is really going all-in on the "kill with kindness" thing, really just turning on all the southern charm and hokum. i find playing that character to be quite funny, so it feels less like i'm being taken for a ride, it's more like i have a private joke.
also, i think it really helps to find some common ground, even for some of the worst people imaginable, they'll usually say at least something you agree with, even if it's something silly, like you both like the same video game - i always start with the common ground, people are way more receptive when you open with "yes, i agree with you, and...", that's honestly been really effective for me.
if you look through my history you'll find plenty of evidence of me not taking my own advice. but i'm trying, and i'm trying to bring others with me
i don't think we all need to be perfect, if we can all just try a little bit