At the moment, I do have two separate stories on the go at the moment, and I've had the same problem as you. I don't know whether it's the right thing to do, but I've found that by putting one down for the moment and concentrating on the other one helps, as I can go back to the other one when I get fresh inspiration. That said, though, I've got a pretty good memory, and I only need to read back a few pages to get back to writing from where I left off. Also, when I do switch between my stories, I prefer to finish the chapter and leave it at that, rather than in the middle of something.
Or at least, that's the way I do it. I'm only an amateur myself.
