[gmku]

I've actually used that with some success. It takes heavy glass, though, and hot sun. I actually unwarped a couple of 180 gram records after two hours in the sun under a heavy glass top to a table (more than an inch thick). Without the weight, nothing will happen. It doesn't always work, and it depends a lot on the severity of the warp and how it happened. Plus sometimes unwarping it creates distortion problems, because now the grooves have been reshaped and may not conform to the way they were meant to be cut.

If it's a slight warp and still playable--doesn't skip--leave it alone, I say. All records, believe it or not, have some warp. I don't have one, but I think a record clamp is a good idea for this reason. They're pricey, at least 50 bucks, but probably a good investment.

I've also found that some slightly warped records unwarp as they sit on the shelf over the years.