Even a small amount of mulch can help conserve precious soil moisture. A thick mulch can suppress weeds and help control erosion, while eventually breaking down and adding nutrients to the soil.
Mulching can also make a big difference in your water bill, reducing your garden's water usage by up to two-thirds.
"Lasagna" gardening is a type of soil building that uses layers of sheet mulch to condition and improve soil. Properly managed, it can be a permanent-mulch approach that reduces time and effort spent in the garden.
An important thing to keep in mind when mulching is to water the ground thoroughly before laying down the mulch -- the mulch conserves soil moisture, but the moisture has to be there in the first place.
An overly-thick mulch, or mulch material that compacts or is impermeable, can prevent water from reaching the soil. This effect is useful when the goal is weed suppression, but is not desirable alongside most garden plants.
A good rule of green thumb with mulch is: the thicker the mulch, the lighter and fluffier the mulch material should be. A compacted mulch resists water penetration and encourages anaerobic conditions beneath; a light, coarse mulch (like straw or bark) allows water and air to penetrate.
Whole leaves, while a favored food of earthworms, tend to compact down to a soggy black layer with dry soil beneath. Leaves should be used sparingly as mulch next to plants, or be chopped into smaller pieces or mixed with other "brown" materials. However, a thick layer of leaves may be useful in open parts of the garden for weed suppression.
While they do not add humus to the soil, inorganic mulches have their uses in the garden.
Sheet or roll plastic is used as a water barrier, to hasten the warming of soil in the spring, and to suppress weeds. (Conversely, clear plastic can be used to encourage early sprouting of weed seeds so that they might be dealt with before planting crops.) Clear or black plastic mulches may also be used to deliberately overheat a patch of ground to "sterilize" the soil of a disease or pest.
Gravel is used to mulch areas subject to erosion, in garden walkways, and to reflect light back up to heat-loving plants.
Flagstones and concrete pavers are sometimes used under the vines of melons; the pavers suppress weeds and the reflected heat helps the melons ripen evenly.
Reflective mulches like bright paper or foil bounce light to crops and also serve to confuse and repel some pest insects -- when they fly over the reflective surface they think they are flying upside down!
In areas with heavy rainfall or where overhead irrigation is used, mulches may aid in splash prevention, keeping soil organisms from being splashed up onto plant leaves where they could cause disease.
Old carpet, flattened cardboard boxes, or thick layers of newspaper, while they may be unsightly, will block weed growth in any area they cover. (The cardboard or paper must of course be somehow anchored against being moved by wind.)