Drip torches are a great tool to light prescribed fires. They are relatively light and they allow a person to be more mobile than with many propane torches.
Drip torch fuel is a mixture of diesel fuel and gasoline. Gasoline helps carry the flame from the drip torch to the ground, diesel fuel provides a longer, residual burn time.
When mixing fuel in drip torches, use 50% diesel fuel to 50% gasoline, in average (mild) temperatures. When temperatures are hotter, bump up the ratio to 60:40 or in very hot conditions, 70:30. This is done because when temperatures are high, gasoline tends to flare-up or burn too quickly, so increasing the ration of diesel fuel helps to keep the flame lit longer and reduce flareups.
When lighting at a fast pace such as from a vehicle, use a higher diesel mixture as well. This will make sure the flame is continuous. When in doubt, use slightly more diesel rich mix.