I bought a pair of good polarized clip-ons last year and would definately
recommend them. They are made by Nu-Clip and cna be found at certain
optometrist shops (if you can't find one e-mail me and I'll tell you the
shop where I got mine). They cost about as much as a good pair of sunglasses
($80-$100) but are very comfortable and custom made to fit perfectly to your
prescription glasses. They were the only company that I found that made
custom clips but Orvis offers good clips in a couple of sizes. If you're
lucky and have the right size specs they're much cheaper.