Basting glue.

When I happen to come to a seam that are pressed the same way I simply clip the seam and make them nest as they should. Be careful not to clip to through the seam :)

If you press seams to one side that side will have four seam allowances, not two. Pressing the seams open makes a lot of sence, unless you are hand stitching.

With 20 pieces in a block it's gonna be downright difficult to get all the seams pressed correctly for them to nest. But I do as ghostrider does and make a careful plan before I start. I am a handquilter and really don't want to quilt through seam allowances if I can help it. Besides, pressing 1/4" seams open would make me crazy! Occasionally it is worth the effort if you have many points coming together in the center of a block. Then I might press one or two seams open. Generally you can find a way to get "most" of them working though. It's just important to plan ahead.