I don't use them but I've heard to never get a flat one and I've heard good things about the turtle shells for grappling. They're these shells that fit into a sports bra--slide around a little but I hear are pretty good. Also, there's a whole thread on this on my friend's forum--it's a kickboxing site and I didn't actually read it but check it out, there might be something:
http://message.axkickboxing.com/inde...20075620.23196