According to Tim Carter from Ask the Builder, "the soil upon which you will pour the concrete must be well-drained and compact." Concrete is porous so you need to erect a concrete slab where drainage is not an issue. Subsurface water can cause the ground the flex, creating stress cracks in the cement. Properly preparing the subsurface helps to limit the potential for cracks in the finished concrete slab. Preparation is the most important aspect of pouring a concrete slab.
Dig the ground down to the proper depth. Determine the proper depth by establishing the height of the concrete slab. Add 2 inches to the depth for gravel if the soil is normally moist or has a high concentration of clay. Most concrete slabs will sit 1 to 2 inches higher than the dirt and 2 to 3 inches below the surface of the dirt, making the needed depth of the hole 3 to 5 inches.
Smooth out the ground with the flat side of a rake so that you have a level surface. Fill in any low spots with dirt.
Tamp the ground with a hand tamper or mechanical tamper. A hand tamper is a heavy pole with a flat metal base and two handles on the side. A mechanical tamper is available for rent from most hardware stores and comes with instructions for use. The tamper packs the soil down. The finished product should barely leave footprints when walking across it.
Pour 2 inches of small, rounded gravel for additional drainage needs. Tamp the rock down until it is smooth and flat.