Pinning at rightangles will help. By coincidence I have a blog on the subject if you want to see pictures of the technique.
I leave the pin in until I have sewn over the seam.If it is at rightangles there should not be a problem. If you move it earlier your seam may still move.
The link is
http://quiltaholic.wordpress.com This will take you to the latest blog which covers this problem.