OK, maybe I'm preaching to the choir, but I had a specific problem this week. I needed to mount some TO-220 (uninsulated case) transistors on a metal chassis, without any drilling. After some searching, I found some stuff from Jaycar, for a surprisingly reasonable price. Part # NM2790. It's double-sided tape, thermally conductive and rated for 3,500 Volts and 250oC. I haven't had the opportunity to measure thermal resistance, but the case temperatures have fallen from >75oC to less than 30oC. Quick, easy and reasonably priced.
BTW: Jaycar rate the stuff at 1.6/m-k. I'm not certain how that translates to #oC/W.
I'm a happy puppy and I know my client will be too.