Sidi Rubber Heel Pads
One very nice aspect of Sidi shoes is that they are designed with longevity in mind. Of course, their choice of materials in the uppers of their cycling shoes is always top-notch. And as a complement, the heel pads on the soles of the shoes are replaceable should they get worn down. They're inexpensive and easily replaced.
If you have Sidis, and you aren't a master track stander, you probably wear through these. I'm on my third set. Good to have some around.