When watering rosemary, check the soil moisture first by sticking your finger into the soil. The soil should be almost dry.
Every third time you water pour the water over the rosemary plant. If you water this way every time you may notice a whitish color almost like someone had dumped some flour on it. This whitish color is from powdery mildew.
Keep your rosemary plant away from furnace vents, doors, and other sources of blasts of cold or hot air.