If it's been spray mounted, I use unseal and a lot of patience to remove the poster from the substrate, remove the residue and then remount with mounting tissue. If it's been heat mounted, a hairdryer will loosten the bond most of the time. Start in a corner and gradually work your way in, stop, and start at another corner. Repeat until all four corners are released, then tackle the middle. Try not to lift the poster too high, or you you could get tiny creases that are really ugly and will never smooth out.