Below, Molly writes into Ask the Inkpot about how to get into a story again after putting it down for awhile. It's a great question, and something everyone struggles with. There was a time in my life when I could sit down and binge write a novel--getting out a draft in two or three months by writing every day, all day, and then doing the same thing with the rewrites. That was before the child, cats who need diapers, cats who need t-shirts, and the trail of dead mice that lead from my apartment to the funny farm.
I depended on that momentum, on living in the headspace created by working on a book every day; it propelled me through a book. This was how I wrote. To be honest, I'm still figuring out how to write in the real world. I'm a writer precisely so I don't have to deal with the real world.
I think one way to approach it is like approaching revisions--start small, low expectations, just try to get back in the book. So much of writing is finding ways to trick yourself, after all. Take a chapter (or several) that you have done already and type them back into your document--you'll probably start changing things here and there, and you'll get yourself back into the rhythms and voice of the book. If that's not enough, start actually doing broader revisions on the part you have--character, theme, plot--look at all your narrative threads and work on bringing them out. Write a summary of each scene and what it accomplishes, both in terms of plot and in terms of the development of the relationships between the characters and the character growth. Your job is to insert yourself back in the world of the book, to remind yourself where you were so you can go forward.
Of course, these are just ideas. Anyone else?