Here is a link showing Cutting Corner and Setting Triangles for On Point Quilts
http://quilting.about.com/od/quiltse...ngtriangls.htm
I'm sure there have been discussion on the board about this before... but haven't done a search yet.
There is a formula for figuring out the size of block to cut the triangles for the sides and corners. In my old age, I have to look this up each time I do a quilt on point ;-)