MAKE SURE that you are sewing a quarter inch seam. Here's a great tutorial:
http://www.quiltingboard.com/t-89997-1.htm
Carefully draw the diagonal line corner to corner.
Starch!
Remember that you are sewing on the bias so it's easy to stretch it out of shape so don't pull or tug the block as you sew.
I agree that a snowball isn't the easiest thing to do.