While this article is mainly focused on fisting for women (vaginal), all of the points are good and valid for anal fisting as well
Top tips:
Be patient! Don't measure yourself against a porn video!
LOTS & LOTS of lube! then add more lube!
Be patient!
DO NOT USE numbing agents (including lube with numbing agents in them)! Pain is a bad thing and blood is a bad sign! If it hurts, slow down. If it bleeds, STOP for now!
Be patient! (You don't have to take a full fist the first time!)
Oh, and BE PATIENT!
FWIW: I've been a fisting top since my 20's...