I do essentially what Prism does except I don't use glue. I stick the pin exactly through the stitching of both pieces and hold that pin as perfectly perpendicular as possible. I then place a pin in normal fashion on either side of it as close as possible to the perpendicular pin and then remove that one. It is usually sufficient to get my seams to match up properly. I also press my seams open which, for me, reduces bulk and creates less possibility of shifting.
Good luck.