I said this in the question thread, but, simple as it sounds, the thing that helps me get back into a story the most is to re-read it. From the beginning if necessary, but definitely the most recent chapters. I not only have to remind myself where I left off, but get back in touch with the feel of the story.
Another thing that helps me is to talk to someone else who has read (and, ideally, enjoyed) the story and is willing to talk about it with me. I find this works best with collaborations, when we can actually brainstorm together, but it's also encouraging to hear from readers who want updates. You can bounce ideas off them too.
