Whether you're starting seeds or growing small houseplants, bear in mind that the smaller the pot, the more often it needs watering. However, there's also the risk that repeated flooding will wash out soil, if not the plant itself. A capillary mat prevents this problem while delivering thirsty seedlings and houseplants just the right amount of water.
A capillary mat looks like a thick piece of felt. Cut it to fit the bottom of any watertight container, place it inside and water it until it is fully saturated. (The container's sides should be low enough that they don't block the light.) Then set your potted plants or flats of seedlings on the mat. Make sure that the pots or trays have drainage holes, and that these holes touch the mat. The water will seep from the mat to the soil—you'll be watering your little plants without continually drenching them from above. Keep the mat moist at all times; as the soil in the pots dries out, it will absorb more water.
Capillary mats are available at garden centers and hardware stores that sell seed-starting supplies, as well as mail-order seed and equipment suppliers.