Nipple clamp recommendations please

Mrs P likes to have her nipples tweaked and stretched during sex, especially nearing orgasm. I happily oblige, but she always ends up with sore nipples for a few days afterwards. Unfortunately, she does enjoy having them tweaked quite hard, which is a bit of a bummer.

We would like to experiment with nipple clamps, see if they can improve things.

Any suggestions would be most welcome.

Without a doubt:

http://www.lovehoney.co.uk/product.cfm?p=14050

These are brilliant.

Just ordered these (http://www.lovehoney.co.uk/product.cfm?p=1769) Butterfly nipple clamps using my Oh points. My lovely lady can get to grips with them while I'm away for the next three months. If they work out, we may look into getting some more, possibly with chains on.