I just read an article from a 70s or 80s british magazine (What HiFi or HiFi Answers, maybe) on accessories, which is available at Vinyl Engine. They test various things - mats, weights, clamps, antistatic guns and liquids. I was pleased by how sober and careful the testing was, and their very measured conclusions - no surprisingly, it all depends. They found most of the mats and weights had positive effects on some parts of the frequency range, and negative on others, and so only actual use in a system can really determine value. They did find that the clamp - a triangular plastic thing that looks like a little oil rig - was pretty uniformly beneficial. Having had one years ago, I agree - one of the few accessories that didn't seem to have any negative effects. Unfortunately a friend liked it so much that I sold it to him.