What I found increases the problem, is leaving the ports or rings in situ for multiple days.
I think that the problem is that the lubricating grease can get displaced away from the sealing 'O' ring when pressure reduces leaving an 'O' ring rubber to metal/plastic contact only. When this happens the resistance to the rotational movement required to remove the port is vastly increased as there is no lubrication and as stated it can be very difficult to remove the port (I've actually used two substantial policemen to do so, which worked a treat but policemen are rarely around when you want them ). In my experience, as Craig says, leaving a port or extender in place for many days and dives seems to make things worse - probably by displacing a little more lubricating 'O' ring grease on each dive as pressure reduces and increasing the area of rubber to metal/plastic contact. I have a strap wrench now too for this problem.