Rather than just laying back and letting your man do his thing, I have found it helpful to mentally rehearse the experience before he ever sticks it in. A little pain is ordinary in the beginning, but it if gets extreme, stop! That was when I first penetrated myself while masturbating and never stopped there. My favorite lube is Pjur Back Door silicone lube. Micro-tears in the anus happen pretty easily when you have anal sex.
There are many books and websites on how to fist properly — Fist Me!
Work up to two fingers, then three, until you feel comfortable gripping them — in control, powerful, flexing your butt muscle. The tips in this slideshow progress from beginner bottoms to more advanced bottoms, so if you are just beginning your magical bottoming journey, the advice at the beginning may be more useful to you than those in the back. Gave me some more insight as a woman and helping me consider if I should try it or not! Start out on all fours and let him get behind you.
If you choose to have sex on any drug or mild-altering substance — alcohol being one of them — know that substances can limit your ability to detect pain or know when your butt has had enough.