the way i usuually do it at my shop is initially try all the usual penetrating fluids,the heavy duty ones like rost off ice(by wurth) etc, sometimes i remove the b/b to spray up the seatube, then after it is soaked it might move. but you have to wait for the fluids to penetrate, we spray then leave it for an hour then repeat a couple of times.
when that hasn't worked, the next step is more penetrating oils and bike upside down clamped by the seatpost in a vice and use the frame as leverage to turn and pull/push up and hopefully remove the post, this usually kills the post!
when that doesn't work we cut the post off at about 1 inch above the top of the frame and we have an electric saw that is like a carving knife that we stick down the inside of the remains of the post and we cut a couple of slots in the post at which point ususally things start to move but if they don't we try and clamp the post in on itself (that's why you leave some sticking out of the frame) and this helps get the post away from the steel tube of the frame and you can force an old screwdriver down between the seatube and the post to wedge them apart. time, patience and skill will eventually get it out. although remember to use a saw blade that will cut aluminium and not steel. this way should not damage the frame if you do it carefully.
you will need to ream the seatube after this then fit your new post with copaslip!
i charge £25 and the new post usually.
hope that helps