I've never tried this before, but I thought I might someday if I needed to.
At a doorway or other out-of-the-way location, put a strip of masking tape where two boards meet. Finish right up to the tape, doesn't matter if you get a little on the tape. Pull the tape while the finish is still wet, then you'll have a sharp line to work up to when you do the next part. Don't tape the newly finished part unless it's completely cured, though, or you'll mar the new finish. A break between a lighter board and a darker board would be a good stopping point. I doubt you'll see break in the two finishes.
If you try it, let me know how that works.