I got this advice from my writing professor this year, and I think it's an important concept to keep in mind for any story. Okay, so he didn't put it quite like that (thank god, or I would have spent the rest of the meeting giggling like a 10 year old) but that's more or less the gist of it.
“The balls” refer to juggling balls which are constantly moving through the air. As soon as you stop paying attention to one ball for too long, it will fall onto the ground and the whole trick will be ruined. The same goes for a writer who is constantly juggling plot points. Everything a character does in the story counts. So if you have them meet someone in chapter two, that character had better come back again later in the story. If you drop that ball, then the story will end up looking like a great big mess.
Often when I'm trying to think of what could happen next in my stories, I end up throwing my characters into new situations that have barely been hinted at. What I should be doing is developing the plots and characters that I already have introduced. A good example of me dropping the balls in a story is with my comic Never. The story got away from me and a number of characters started popping into the comic when I should have been more concerned about developing what I had already built.
Generally this is something that can be fixed through planning and outlining. Since I'm more of a fly by the seat of my pants kind of girl, this tends to be the most difficult step of writing for me. However, I've said it before, and I'll say it again, writing is hard work, and you have to suffer through the parts that you find hard in order to enjoy the rest of the process. In the end, that suffering will make the story a whole lot better.