After a few big fish it's not unusual for butt joined rods to get very tight. What you need is to get a good grip on the top section and not damage the rod. One of the best ways(and this is probably similar to what Brandon does) is to wrap a quality rod wrap spirally, neoprene side to blank, around the rod, the soft rubbery part will grip the blank and not slip, this will provide a sure grip on the top section and with another bloke holding the bottom grip of the rod allow you to exert enough force to twist and pull the rod apart. Suitable wraps include ones from Daiwa, Snowbee and ATC.
By the way most manufacturers seem to recommend no substances on the joins. A lot of problems can come from foreign matter on the join surfaces. Once the rod is apart, try jamming a clean cloth all the way into the female section and wiping the inside thoroughly, this often solves the problem.