I've worked in photo technology over a period of many years and have had to remove many, many stuck flanges, some that have defeated other technicians.
A highly effective tool to deal with stuck rings is a rubber stopper. Pick a size that covers the ring, press hard and turn. These can be bought in a range of sizes at hardware stores or from Micro-Tools. Among camera repair technicians, stoppers are the standard tool. I have a whole bin full of stoppers in a wide range of sizes on my workbench just to remove rings. In really tight situations I've machined a matching tube and glued a ring of rubber on it. As mentioned by others, double sided tape on a matching ring or disk is very good as well.
Pliers and pipe wrenches are not helpful, as they distort the ring in a way that actually binds it tighter in place. Ring or filter wrenches that encircle the ring are good, but I've found them generally less effective than rubber stoppers.
To prevent sticking, a very small amount of lubricant on the treads is a good idea especially if the parts are anodized. Anodizing is actually rough/porous at the microscopic level and thus tends to bind. Lubrication, especially a waxy lubricant like Door Eaze Lube Stick, helps a lot. Silicone lubricants work well too.