Practice, practice, practice. For a long time I was mostly a top since getting fucked hurt a lot -- I'd cringe while getting a dick up my ass and then I'd bleed. Not fun for me. Then I had a dry spell and bought some toys to play with. Practicing with a few dildos (one small, one medium, and one monster), I worked my way up until I could actually take my own fist. I discovered that lots of lube makes a huge difference for me. I bought one of those lube injectors so I can lube my hole nice and deep. That way I can leave the house ready for action and not have to worry about drying out. Now I play with toys about once a week and try to get fucked a few times in between. I actually prefer taking multiple cocks when I bottom, since I love it when my ass gets nice and sloppy with lots of cum. Anyway, I still top a lot more than I bottom, so I have to keep practicing with my dildos to be able to get the kind of fuck action I love.