1. If it doesn't fit, don't force it. Doing this can be bad for your anal health, not to mention pleasure. If you ignore all rules bar one, don't ignore this one.
2. Too much lube is almost enough. You need lube, and lots of it to make it work. The more lube the easier things will go in.
3. Take your time. You need to be relaxed to make strapon sex enjoyable.
4. Have plenty of old towels, because there will be mess from strapon sex.
5. If possible, train your butt for strapon sex with a dildo.
6. Don't put anything apart from strapons and dildos designed for ass play up there. After all you don't want to loose anything in there!
7. Make sure your strapon (for both you and the strapon wearer) is comfortable and not too big. Make sure the harness is not too tight, but not too loose. Otherwise the experience wont be as smooth as it could be.
8. Never share anal toys. The reason should be obvious
9. Keep your anal sex toys clean. Wash them in hot soapy water, dry them and store them in clingfilm till the strapon sex session.
10. If a toy breaks, don't be cheap and try to repair it. Bin it and get a new one. They are cheap enough and you don't want to risk it.