To be honest I find it helpful to not bind yourself to one work at a time. If you do that you might find one incredibly cool storyline, and instead of letting it become its own work you'll drag it into a story where it might not fit as well.
Plus what helps too is to bring in everyday experiences into your manga. Like right now I do a keenspace comic that centers around the adventures of my three friends and I. I almost would want to eventually make that more manga-esque, but right now its flowing better as a free form monster.
Also go to Borders and read lots of mangas. That helps. Immensely. Even if you don't have writers block. The new Hana Yori Dango is out, that;s reason enough to go.
Oh, also, I was curious if anyone else here had online comics? I kind of want to start reading more than just the few really popular ones I read and the ones my friends I knew from before write. o^-^o