I have a bike with a seatpost stuck in it. The top has been pounded out of shape so I can't get a seat on it to take it out. Any suggestions?
You have to grab the post somehow and twist it to get it out. Be sure to loosen the seatpost clamping nut/bolt all the way. If there's enough of the post sticking out, and the post is a constant diameter, you might be able to cut off the damaged part of the post with a hacksaw and attach a seat to the post. With a seat fastened tightly you'll have something to use to try to twist and pull the post out.
If that doesn't work or you can't get a seat on the post, I would try using a Vise Grip tool. If you heat the seatpost first with a propane torch that will make it and the frame expand slightly and this can help a lot, but be careful not to burn your paint (wrap a wet rag around the painted areas to keep them cool and heat only the post, not the frame). Be patient and let the heat do its work for a few minutes and then try to twist and pull the post out. If you have a bench vise, you can also hold the bike upside down (get a friend to help you), clamp the post in the vise and then turn the bike to try to get the post out. You don't want to break the post though so don't overdo it. Rock the bike sideways instead of trying to turn it too far on the post. You're trying the break the corrosion first and then the post will come out.
You can also try squirting some liquid wrench on the edge of the post right at the frame and tapping with a hammer on the post to vibrate the post and get the liquid wrench in there to break the corrosion, too. Liquid wrench takes awhile to work. You might have to apply it for several days and keep trying. Sooner or later, though it will break the corrosion and you will be able to extract the post as long as you can get a good grip on it. But, because it takes time, I usually try the vise grip and heat approach first.
If you break off the post when working on it, you can usually slot the back of the remainder of the old seatpost and insert another post that fits inside the old/stuck post as a way to fix the problem. Or, you can ream the old post out, too, if you have the tools.
Hope this helps you out,