loosen the holding bolts, make sure that the only force preventing the seatpost from coming off is the seatpost itself, ie. loosen all bolts around collar, QR clamp, etc.
go to a hardware store and buy yourself a spray bottle of "penetrating oil"
spray generously around the collar and leave out in the garage overnight. in the morning, try rotating the post, and if that doesn't work, put a piece of 2x4 over seat and hammer down, loosen whatever is sticking, and pull the seatpost out.
I work in London, Ont, so I see a fair bit of this going on. Once you have it off, if you have it off, lather on white lithium on the post and put it back in.
Upon closer examination of your post, for those instances where the seat has come off, we use Canadian Tires rubber belted wrenches. We have once drilled through the seatpost horizontally and put a rod through that, by utilizing a couple, we were able to pull the sucker out.
Last edited by TriEngineer; 03-22-09 at 08:44 AM.