Try using a speculum during play at home.
I use one every so often, and it's all a matter of relaxing when it goes and and opens up.
I don't think that it matters on how tight you are, you can be stretched and it's a surprisingly good feeling to have a hand up there.
I've yet to hear of someone who's been "stretched out badly" through this type of play. I would think you'd have to keep a fist up there all of the time for that to happen.
Lots of lube, and a latex glove will help you (unless you have allergies), if you opt out of a glove make sure his fingernails are trimmed and filed, and no hangnails (I've had a jagged fingernail up there, and it isn't very pleasant).
The key component is to relax.
Relax your mind and your body, breathe out deeply through the insertion.
And of course it's all a matter of going slowly and gauging your comfort level by inserting a few fingers at first and then working up to the whole fist.
The vaginal fisting seems pretty easy for me, since after that, I'm going to try anal fisting next.