When I get stuck on longer works, it's usually because I've gotten to a point where something needs to happen but I can't think of what, or I know what needs to happen later but I can't figure out how to get to that point. (Sometimes, I just deke around this by writing something like "Three days later, they arrived at the harbour." Of course, this presupposes that the story will still make sense later if I elide those days, and that it will not look noticeably lazy because I've skipped over events that needed to happen to get them to the harbour. I am not always the best judge of what events need to be there for later events to occur, and often end up going back and patch-welding them in.)
Anyway, here's a list of the ways I get myself unstuck or jiggle an idea loose!