Regarding stuck aluminum seatposts and stems, I have had some success judiciously using a propane torch to heat up the "frozen" area. The reason the seatposts and stems get stuck is because electrolysis takes place between the dissimilar metals. Steel and aluminum have different coefficients of expansion, though, and a torch will sometimes work when all other attempts (twisting, pounding, use of penetrating oil, etc.) have failed.

The problems with using a torch, of course, are the almost certain need to repaint the torched area and the risk of warping the tubing or weakening brazed joints. You really have to use a light touch.