They are sensitive to the quality and temperature of the water. Ideally, provide room temperature distilled water. Keep them a little drier when they aren't in bloom. Do not allow gardenias to sit with water in the saucer for more than an hour or two. Gardenias bloom their best with warm days and cool nights.
Gardenias require high humidity to perform their best.
Use a humidifier or a humidity tray indoors, especially in the winter. Feed gardenias every two or three weeks with a water-soluble fertilizer such as Bachman's Excel Gro Plant Food. Several times a year, use an acid fertilizer. If you are not using distilled water, be sure to use the acid formula on a regular basis. Use a high quality peat-based potting soil such as Exceloam.