Ironically, the number one reason for plants not thriving is overwatering. Some plants love moist soil, but most—especially succulents and cacti—prefer it when the soil dries out between watering. A general guideline is the thicker the leaves, the less water needed. Feathery ferns need more water than turgid succulents, which can get away with a monthly watering.
To water your plants, lift the leaves and pour directly onto the soil until you see a little water trickle from the bottom. Water caught in the plant pot saucer should be dumped after 30 minutes. Plants love a nice shower, not soaking in a bath.