They're not crazy about change and can be picky. Once you get one that is happy, they aren't hard to get to bloom. (Though you can put them outside in the summer, if you have a spot they like.)
To have them flower, gardenias must have steady temperatures while they are forming buds. Highs or lows make them drop buds. They like it between 60 and 75 degrees most of the time.
They also want humidity. Set on a tray of pebbles in water.
Keep moist. You can let the top 1/2" of soil dry out between watering in summer which is when they normally bloom. Do not use chilly water.
Feed with an 'acid'fertilizer every two weeks during the growing period March - September. No lime.
Lots of sunlight, but not direct sun.