Plumber's tape or teflon tape as others have properly described is great for mating any threads. It acts as a good sliding surface between threads, it tightens up sloppy threads and it will help with the dreaded creeking noise that you get on some alloy bikes. You can use as little as one wrap or as many as you need to build up to the desired thickness.
A few years ago I purchased a bike that had all clamp on components from an older gentleman. The paint on this bike was fantastic and when I went to remove the front derailleur and cable clamps I noticed that there was something between the clamp and frame. It was a few mils of teflon tape. When I unwrapped it, I noticed it protected the frame so you could not even see a clamp mark. It was invisible in use as it was pulled away from the edges presumably using an exacto knife and a lot of patience. I will always remember this when I use clamp on components.
Also in the spirit of not wasting anything, I find that used teflon tape make a great sugarless chewing gum.
Ray Homiski
Elizabeth, NJ USA