Try penetrating oil instead of WD_40, like PB Blaster, look for it at an auto parts store, Liquid Wrench would be a second choice. If you can get to it from both ends, thorugh the BB, and from the top it may help. Twist the post with a pipe wrench, use a cheater bad on the wrench handle, something will give.
Anyone have any ideas on how to remove the post from the frame?
As already mentioned, first put the saddle back on the post. If you care much about the saddle you took off, you might use a junker saddle instead. The method I've used successfully in these cases is to put a junk saddle on and use hammer blows to the nose of the saddle, first one side and then the other. This has always worked for me, but it's clear from reading many posts regarding BADLY stuck posts that I've been fortunate and not had to deal with a really bad case. The source of the problem is more than likely galvanic corrosion, which is usually a result of an aluminum post in a steel frame, particularly one where grease wasn't applied when the post was inserted. Regardless, if the hammer-on-nose-of-saddle method doesn't work, here's Sheldon's article on the subject:
I've worked on it for awhile today and it hasn't really moved.
I put the seat and saddle back on and beat it left and right with a hammer, hitting the front-most point. After looking at Sheldon's website, it looks like the past owner might have hammered it into the frame.
Ouch, if they really hammered a too large post into the ST, then you're probably going to have to cut it out with a hacksaw from the inside. Do a search on 'stuck seatpost & Laprade' to find some recent threads on this.
I can't imagine that someone would go to the trouble of jamming a too-large seatpost in. That's a lot of work, when cheap seatposts abound. I think lots of PB Blaster and heating the seatpost (assuming you don't care that much about it) (don't heat the seattube), combined with a BFH to the saddle should do it eventually. The last resort is the hacksaw, though. If you haven't looked at the Sheldon link above, check it out.
I had a good friend destroy a frame by applying too much force trying to get a stuck seatpost out. I'd take it to your LBS and see what they can do with it. Zinn has some last-ditch suggestions in his road bike maintenance book too, if you're desperate (desperate as in "remove the blade of a hacksaw, wrap one end in tape...").
Drill a hole in the seatpost big enough for one of those mammoth screwdrivers.
Pack the seat tube and post in ice and get some water boiling. When the water boils, pull the ice packs, pour the water over the seat tube ONLY (not the post), put that screwdriver in there and twist hard, with both hands.
If that doesn't work, you'll have to use a loose hacksaw blade on the inside as suggested above. cut the post off about 1/4" from the frame and start cutting. Sucks though.
What I might try is attaching chain to the seat post (maybe drilling hole through post and attaching bolt). Attach the other end to something heavy. Then after getting as much wd-40 down around the post, hold the frame down, and try repeated jerks to pull the post out. Kind of like a reverse dead-blow hammer. Try to get more oil down there.
If you can get the thing to move at least a little, you got a shot.
If you can get some oil down through the seat post, let it run down into the seat tube, then turn the bike upside down to let it try to get in around the seat post from the bottom.
After injecting the post with ammonia via a syringe from the campus nurse, I threw it up on a vise and twisted the frame off the post. The guy at Evergreen Bike Shop in East Lansing let me use his vise, and we were able to drop it in about ten min.
I figure the old owner dropped on a 27.0 when the shaft looks like a 26.8. The shaft was all brown with rust, and there was a pile of it underneath the bike when we schwacked it with a rubber mallet. I picked up a nice post and an in the market for a more comfortable seat, but otherwise, I'm back in business.