Succulents are desert plants, and have evolved to survive in areas with extremely limited, infrequent rainfall. Their thick, fleshy leaves look that way because they store water in them rather than depending on external water sources. If you break off a leaf, you will notice a snapping noise as well as oozing sap that looks like mucus. This is what protects succulents from drought and lends them their abilities to go a month or more without water.
Succulents are very susceptible to overwatering, especially when gardeners unfamiliar with them try to treat them like ordinary garden or house plants. However, they are also at risk for underwatering in the hottest months and in the brightest light, and this is what leads to wilting. When your succulents lose their rigidity, or their leaves start to look colorless, yellowed or gray, they may not be getting enough water.
Any time a succulent looks dry, it needs water. Avoid overcompensating, however, because this will simply lead to root rot and drowning. "Water your succulents" sounds easy enough, but it is important to take into account the needs of the plant based on its environment at the time. During the hottest months, it is important to pick up your watering schedule so they do not wilt. Slow down again when weather cools and sunlight lessens. Water just enough to moisten the soil around the base of the plant, but do not soak the soil.
One of the easiest ways to keep succulents happy is to provide the correct growing conditions for them. By potting them in very well-drained soil in the ground, or in rocky potting soil in small pots, you can ensure that moisture is wicked away quickly so that plant roots do not rot. If you wish to take clippings, do so in the early spring, just as succulents come out of dormancy and begin to grow again.