The best way is first to read about the species of plant that you're growing, to make sure you understand what kind of conditions it likes and thrives in. This is probably the most important rule of all, at least IMO. Plants are living creatures, just like animals, and they have specific and individual needs that must be met, if they are to grow and thrive.
Secondly, dig one of your fingertips into the soil about an inch deep. If the soil is dry, the plant probably needs some water. There are exceptions, however, such as cacti and succulents, which prefer to dry out even more between waterings, or bog plants, which prefer to be kept very moist.
Thirdly, if you know you're prone to over water your plants, use unglazed clay pots -- rather than those made of glazed clay or ceramic, or plastic -- because they allow water to evaporate from the root ball a little faster. Always make sure your plants are in pots that have drainage holes in the bottom, and never leave them standing with water in the saucer or drainage tray.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams