The obvious answer is to not let the nipples get stuck in the first place. I ALWAYS use an anti-seize material.
But if the gun came to you with stuck nipples then good luck. I have an ASM 1860 that I was determined to get the nipples out of it and wound up ruining the cylinder. I tried heat-cold-brake fluid-WD40-laquer thinner-and all of the various rust removing or rust loosening solutions over a period of weeks and one simply would not come out. I used every type of nipple wrench I could find and made a few from sockets and broke or bent them all. I finally broke the nipple off at the threads. I then tried to drill it out and the drill walked over into the body of the cylinder and ruined the threads.
The cylinder sets in a box to remind me to put the anti seize on the threads every time.
The remnants of the nipple are still stuck in the demolished cylinder.
A note: Even with the anti seize material on the threads I do not tighten the nipples. I snug them up until they stop turning and quit there. I guess you could call them finger tight but no more than that.