I don't know if you have a vector based program or not, but this is what I do...
First I make the shield shape then I clone it and cut it in half.
Then I take the left half, make it white and change the transparency to 80%. I trash the right half.
Place my image or text on the logo and the transparent half over the top.
So what I have is what looks like light coming from the left. So I need to have a shadow on the right. If you add shadowing, you need to account for the logo size change, so just shink it down some. I usually make my logos at 148pix with the outside 2pix transparent.
I hope some of this is helpful. Anyway, this is what it looks like in the end...
