Burner plates, also called simmer plates, fit right over the burner and help to diffuse the heat. This gives you a wider range of temperature control and also helps prevent hot spots in your pan.
The best burner plates we've used are fairly thick and made of either cast-iron or steel. Plates made of thin aluminum are cheaper, but we find they don't do as good a job of taming the heat. In a pinch, you could use a cast-iron skillet as a heat diffuser and set your cooking pot inside.
You can find burner plates at most stores that carry kitchen supplies or on the web:
Do you use a burner plate?
Related: Top Ten Most Useful Kitchen Gadgets