This is the post I just put on the "sashing" question today:
One thing not mentioned so far: if you have to put a seam where there shouldn't be one, be sure and press the seam open. Seems like a little thing, but a seam pressed to one side has a little bump. When it's open, it's almost invisible.
This is true for borders, too. And if you have to add a little piece to any parts of your blocks, open seams are hard to find, even if you know they are there.