I had exactly this problem and fixed it a few weeks ago by pouring boiling water on the outside of the frame so that it expanded (this worked fine - I think you need to get the metal to hold the heat for a bit, not just get it hot for an instant). But if that doesn't work, I've heard of everything from taking the bottom bracket out and pouring cola down the inside, to using a hacksaw on the seatpost.
In my case it turned out to be galvanic corrosion building up inside the frame, I suspect caused by a build up of dampness after wet rides.
After I got the post out, I did quite a bit of scraping with a wire brush and screwdriver but some of the material wouldn't come off. I put some carbon fibre assembly paste in to hold things but I don't know if copper-slip would be better to prevent the seizing - I've heard that it also can react with the aluminium. Even so, the seatpost now almost glides in and out compared to before it got stuck, so I will definitely be checking inside the frame at least every month.