Bicycle seatposts are notorious for getting stuck in the frame. They're in a fairly vulnerable part of the bike, and rainwater can work its way down - causing the post to corrode over time. The act of corrosion is fairly slow, so you should only need to check it every few months by loosening the clamping bolt and giving the post a wiggle; any resistance and the post should be removed and cleaned. If the post won't move though, you'll need to put more muscle in. Before you continue though, do so at your own risk! I've never broken a frame before, but freeing a stuck seatpost can be a fairly physical experience. The saddle and seatpost could also get damaged - just to warn you!
Remove the saddle and wheels, and clamp the seatpost in a workshop vice. See if the post comes free by twisting the frame back and forth. It can be sufficient to just wiggle the post out with this extra leverage, but not necessarily. For a particularly stuck post that barely moved in this position (and certainly wasn't going to let itself be pulled out by hand) I had to resort to a solution with a lot of tension. With the wheels off, I looped tough rope around the frame and an immovable pillar, and looped more tough rope around the seatpost and another pillar. I then span the bike around in circles until the rope was really taut - and then started turning the seatpost back and forth. This was an alloy seatpost in a carbon-fibre frame, and the first wrenches made horrible cracking noises. However, the tension was causing the post to be slowly pulled out - and by slowly, I mean around 1mm for every 2 minutes of wrenching the post back and forth. To get enough leverage I actually had the saddle in place, and jammed a solid metal rod (12mm diameter, 30cm long) into the saddle and wrenched the seatpost using that. This was fairly trial and error, but the post did eventually come out. White deposits indicated that the alloy had indeed corroded, which had caused it to expand and get seized into the frame.
After all this the saddle, post and frame were all fine - but the post had got a bit chewed up in the vice.