Everyone is different of course, but what worked for me/us was:
1) Verbal persistence. I told him how much it would turn me on for him to try it on me and then once I got him there, I told him how much it would turn me on for me to be able to try things with him.
2) Taking it SLOW. It took years before he'd try it with me and another few before he'd consider it. But now he's warmed up. He'd never suggest it, but if I do, he's open to it now. THANK GOD!
3) Not spending too much time right at the opening. It was serendipity really, he was saying that he had some discomfort with a very, very small butt plug and I sneezed when it was just a 1/2 in or so in and it went all the way in and he immediately breathed out and said it felt so much better now that I was past the sphincter! So now we don't waste time at the door, but just go on in once he is lubed up! Try the shooters to get lube in there first, too!
4) I've learned to not focus on his phenomenal ass too much. I will get a butt plug in and leave it and focus on a blow job or letting him play with my tits while I mount him from above and thrust against him without any harness or dildo present and sort of mimic it, but with it stable in his butt. Then we move on to the harness/dildo or sometimes just the dildo.
5) I buy small, slender toys for him and larger ones for me. I find that he's comparitive and mentally things it is okay because while he might normally thing something is 'huge' for his ass, when he sees it in comparison to what I use, he thinks of it as small and easy.
6) Before, during and after I always am sure to tell him how hot it makes me for him both to recieve and to give and that helps...a LOT!