How can I become a pro in anal sex?

All my boyfriend wants to do lately is anal sex. We've only been able to do it successfully one time and we were both really drunk. The second time we tried it, he was able to get halfway in but then when he tried to go farther it hurt so bad and it made me want to stop right then and there. He really wants to do it more and I really want to learn how to get through the painful feeling it kinda gives you. PLEASE help me out!