Add organic material to the soil on a regular basis. The greater the variety--such as leaves, vegetable and fruit waste, and animal byproducts including manure and fat--the richer the humus. Compost is already mostly decomposed and helps the soil create humus much faster.
Avoid fast-acting chemical fertilizers, which often add too much salt to the soil for the microorganisms to survive. The loss of the soil-dwellers greatly slows down the decomposition process.
Keep the soil moist. Dry soil loses the ability to decompose its organic material. Lay a protective mulch over the soil after the soil has warmed in the spring to keep the moisture in.
Avoid disturbing the soil by tilling or turning it over. The complex web of decomposing microorganisms depend on a stable environment. Disruption of the soil forces the soil microbes to regroup and start their efforts over again.