This tutorial will work for hexagons or diamonds, the theory is the same, stopping/starting 1/4" from the end of each seam line. You can try clipping the diamonds like Patrice did the hex's to see if that helps them lay flatter :D:D:D
http://www.quiltingboard.com/t-3419-1.htm
Here is a tutorial showing how to set in a square using Y seams
http://www.quiltingboard.com/t-34066-1.htm