The ideas above are all valid, but I'd just add that stripping the bike completely and clamping the seatpost into a vice, then turning the frame, gives a lot more leverage. Don't pull on the chain / seat stays by themselves, or you may end up with 60mm / 220mm rear hub spacing and a bike that rides like a crab.
Obviously, this assumes that there's something "clampable" on the seatpost. There often is, but even if not that seatpost is probably a goner anyway; consider creating flats by cutting. The trouble with using the saddle as a lever is that it absorbs quite a bit of force.
As a very last resort I have sometimes had to cut the seatpost off a little above the seat lug and then cut (carefully!) through the post, out towards the frame. You can generally tell when you've hit steel! If you've left enough it's sometimes then possible to get things moving by squashing the post and breaking the chemical bond that's holding it in place. If not, keep on cutting until you can hammer and prise the slices out...horrid job, but you'll get there in the end. Last resort, though.