I'm going to try to explain my process, but if you need further info you can Private Mess for more.
With right sides together stick a pin through the two pieces at the exact point of intersection. Then lift the top layer and...with the pin still in place...put a tiny spot of basting glue at that point. Then using your little Clover Iron hit that spot with heat to dry it, and move on down to the next point of intersection and do the same. You can use a big iron, too, but those little tiny pointy ones are perfect for this. By doing this you will eliminate your pins which are a big reason why the fabric is shifting. I even run a VERY thin bead of glue down the whole seam keeping it just inside of the line, and use no pins for seam lines. If for some reason it isn't as good as you want it...moisten the area just a little to be able to separate the pieces and go again. Make sure it is basting glue or Elmer's washable or you won't be able to have any "do overs." Hope this helps.