Clean yourself out first. Get a good enema. Then make sure you're really turned on--literally wet everywhere. Lube up your finger, start slow and slide it in. If it hurts, you're doing it wrong. Hold it there for about a minute, or even longer. Try two fingers (make sure to lube them up!) Again, hold for a while. Then three. Finally, lube up a butt plug, put it in. Hold it for a while. When it truly becomes time, your butt should be ready.
When it comes time for him to put it in your butt, make sure he's very lubed up. Have him remove the butt plug, then have him slide in his cock very very slowly. Don't jam it in. Don't thrust away. Once he's all the way in, make him hold it there for a minute. Then, start thrusting away slowly. Again, if it hurts, then you're (or more likely him) doing it wrong.
Anal is not something you just jam your cock in and thrust away. Dumbasses only do that. It's why there's a lot of injuries (think "pink socks") that come from that. You have to prepare. It's not a spontaneous event. You have to prepare.
