Amy, if you try the Superfeet, be sure to get the kind you need--they come in two versions, one for heels and one for flatter shoes. It depends on the height of the heel as to which one you get. Nordstrom has them and can help you find the right kind. The Superfeet are a rigid support and are 3/4 length. I use them, and whether they fit in the shoes really depends on the amount of space in the shoe.
I have tried the Walking Company ones recommended by Tarzy too. They felt really nice and seem like a really good product, but they took up quite a bit more space than the Superfeet, and so I wasn't able to use them in my shoes. I don't know if sizing up half a size in the shoe would have fixed the issue. These insoles too are very rigid and seem like a really good product. I would definitely have kept them if they would have fit in my shoes.
Chiropractors usually deal a lot with insoles. If you have a chiropractor you trust, you could ask him or her about it. My chiropractor carries a brand called Foot Levelers, which I have never tried--and I don't know if they have a version for heels. She said they didn't take up much space and that Foot Levelers has a sale going on this month. However, I think they are still pricey, in the $200-300 range.