Use lots and lots of lube.
Be completely relaxed and with a partner that you know you can trust 100%.
Don't tolerate any pain. Stop if it hurts.
Use gloves on hands and make sure nails are short.
If you're talking anal sex (or similar sized toys) as a goal then work up to it over as long as it takes. Plan on a couple of weeks, anyway, maybe even a couple of months. We (people in general) generally condition ourselves to "keep tight" down there, and it can take a while to learn how to relax.