It's about the same answer every time in SLR.
COMMUNICATION!
This is what you do. Sit down and watch a porn with the type of anal that you're into with your boyfriend.
Whether the whole deal is about anal, or it's spontaneous anal ( My favorite type ) using your own juices as lube for your ass
When you see what you want in the porn, you turn to him and tell him "I wish you'd do that with/to me"
That's exactly how he wants to here it. Trust me. He's got his own horny ideas, he's just looking for your approval.
Personally, I just try it and see what a girl says. I'll usually eat her pussy and finger her, then slide a finger behind and play with her ass and see what she says. If she seems to like it, I'm probably going to put it in her butt that day