Frost heave occurs in concrete slabs when water in the soil underneath the concrete freezes, displacing the soil and in turn heaving up the concrete. This process forms unattractive cracks and hills in the slab. Once the concrete is laid, you cannot do anything to stop frost heave other than wait and let the soil and concrete settle. The key is to take measures to prevent frost heave before you lay down the concrete.
Excavate a foot into the ground and compact the soil with a soil compactor, which you can usually rent from hardware stores. This will prevent water from penetrating into the soil as easily and provide a solid base for the concrete slab.
Lay down 6 inches of crushed gravel as a layer between the soil and the concrete, creating a porous area from which the water can drain. Place a 4-inch perforated pipe in the gravel and route it to a dry well, or just outside of the area where the concrete slab is located.
Pour the concrete slab until it is about 3 inches thick. Lay down ½-inch steel reinforcing bars 2 feet apart from one another and then a fibermesh slab. These will prevent shrinking and cracking and strengthen the concrete, making it less susceptible to frost heave. Pour 3 more inches of concrete on top of that to finish off the slab.