Actually some of these are stuck really, really well. And if they are stuck that well you can end up collapsing the tube and shearing it off at the top as you try to remove it. Then you are really screwed.
The best way I've found is to find a bolt or round steel piece that fits very snugly in the tube. Heat the cast iron with a propane/mapp torch and if you are lucky you don't have to break out the oxy/acy torch and really heat up the area around the tube.
Grab the tube with round jaw vice grips and they usually twist out. If you have to get pump really hot with the oxy/acy torch you may have damaged your bearings/seals. Then you wish you had just paid a rebuilder to do all of this for you.