All the advice here is great, but there is one piece that I am surprised not to have seen.
I have had better luck when the receiving person, more than relaxing, actually bears down like they are, for lack of a better phrase, taking a shit. This, of course, means that you will want to be sure things are empty and clean down there, maybe even had an enema, but there's a reason doctors ask you to bear down when they do an exam back there. It makes for easier access. So, combine the bearing down with the lube, the desire, the practice (gradually increasing the size of buttplugs is a great help), and all the other advice here, and you should be able to make it happen.
The girl I currently assfuck had never done it before we tried. Now I think she prefers it to vaginal. And it all started with the advice to "Shit it open." You had to be there I guess.