Most plants need to be watered when the top centimetre of soil in the pot is dry to the touch. Plants adapted to arid regions, including cacti and succulents, can be allowed to dry out even more. They should be watered more frequently in summer, however, when growing conditions are optimal.
Avoid watering too little, too often. Thoroughly soak the soil surface until you see water coming through the drainage holes, and remove any excess water sitting in the saucer a few minutes later.
Use room-temperature water. Cold water can slow the growth and produce spots on the leaves of various tropical plants. In addition, because some plants are sensitive to the chlorine in tap water, letting the water sit for a day before using it will allow the chlorine to evaporate.