Always had good results with PB Blaster. Forget about WD-40. I had a stuck SR Laprade seatpost in a Specialized Rockhopper Sport. In there very well. 24 hr. soak while inverted, followed with a big wrench and me standing on its end, forced a small turn. That's when I knew I was going to be successful. It has managed to break free BB's seatposts, suspension bolts and tie rods, where Liquid Wrench had failed.
Those in Europe who can't get PB Blaster can try Kroil. Heard this stuff is like magic.