Hey David,
When I think I'm done with a book (I write novels), I save my money and send it to a paid editor. I have several I use, depending on the material. When the manuscript comes back, I usually agree with some things mentioned to change or fix, so I fix or change them, and send it off to my agent. If she wants changes, I do that, too. Then I'm done. By that time, I a SO READY to ditch that story, there is never a problem of wanting to go back. My toughest thing is getting a new novel going that I find interesting enough to keep going. But I keep trying until I succeed. Rejection is the constant meal of fiction writers. :). Good luck!

Hi David,
Jack's (really good) answer above is very close to mine. I'm normally so happy that the one I've been working on is finally done, that I'm more than ready to close the door on it.

Generally speaking, though, I'd say the trick is time and emotional distance. That is, taking a longer break between projects, clearing your head of the last story by doing other things besides writing or editing.

Writing tends to stick us in a bubble, so you do whatever you can to get out of it, get some fresh air, and come back to your writing desk when you feel refreshed and ready to go on, not before. For me, that's normally about 2 months.

Good luck and have lots of fun with your new project!

Emotional distance is a great phrase and absolutely true. I don't think about emotions enough. As an old newspaper writer, I keep my head down and fill up that blank paper. I think I'm like, "stop thinking how you feel and get the job done." Ha.