I would have to agree with Inquisitor on this, foremost. Then, I would learn all you could about anal pleasure. The biggest mistake would be for either of you to go into it blind without a little bit of knowledge base. Been there, done that, got the t-shirt. Like you, the experience was not what it could have been. It's important that he knows how to make you feel comfortable and relaxed. Then, it's equally important that you trust him to not hurt you and know what he's doing
I would suggest going slow (maybe a finger) and lots of lube. Lube is key here, it's your friend. When I was preparing, I got a set of anal plugs in a couple different sizes. I would wear them for short periods just to get used to the size and work myself up to the next level. It made actual intercourse a lot more pleasant when the time came. A trick I learned, sorry if this is TMI, it could be useful, try "pushing out" a little as he is "pushing in". This will open your cavity for an easier fit. Then you can relax and enjoy.