There are skinny rulers that help do this for you. You center the points of the fabric on the center of the ruler, and mark either side. Sew on the lines, then cut in the middle. Works great! I think I got the Fons & Porter ones, but I'm sure others must be out there too. This is what they look like:
http://www.shopfonsandporter.com/pro...il/FPR7845.htm