Here's what we do with very light frames - it seems to work OK with any frame and post material - including carbon.
Wrap the affected area of the frame (upper seat tube) tightly with cloth (such as an old towel), and leave for an hour in the coldest environment possible. Then soak with three or four continuous kettle loads of boiling water. This usually causes the seat tube to swell by a sufficient amount to break the bond. The post should twist out using only the saddle as a lever.
In our experience the 'saddle off, clamp and twist' method can damage (and unfortunately has ruined several) lightweight frames. This method should only be used once the bond has been broken, and the seatpost has begun to move. Sawing a post out is also a very difficult method, as it's difficult to avoid cutting into the frame.
If an alloy seatpost has electrolytically corroded into a steel frame there could be a considerable bond, due to the aluminium oxide effectively swelling the post. In such cases there is only one option available without using a flame. That is to dissolve out the post using frequent changes of caustic soda solution poured into the inverted frame through the BB. Caustic soda is highly dangerous, and will cause very serious burns if it gets on your skin. You can get it from any hardware store, but do be careful - and follow the safety instructions to the letter. The solution agressively dissolves aluminium, whilst leaving steel unharmed. Test the solution with an aluminium drinks can, and be amazed as it 'boils away' to nothing in a matter of seconds! The alloy post should be sawn off so that only about four inches (10cm) protrude from the frame, and then tightly plugged with a cork. It should also be internally de-greased as much as possible before introducing the solution into the seat tube. BE VERY CAREFUL!
Oh, there is one more method. Travel back in time, and grease the post/frame interface properly - with regular removal and re-greasing during monthly maintenance sessions.
STEAL WON!