The process of setting concrete is known as curing. Though many people use the term "cure" interchangeably with "dry" when speaking about concrete, it is not advisable to simply leave poured concrete out in the air until it hardens independently. Curing is, in fact, a more active process than drying. Curing means keeping the outside of the concrete moist as the inside hardens, so that it is as strong as possible. Therefore, to speed up the setting of concrete is to speed up the curing.
Leave the formwork of the poured concrete in place as long as possible. It will act sort of like a dam and hold in a bit of moisture. Do not remove the form as soon as the edges of the concrete have set, since these will be the weakest areas if they dry out quickly.
Cover the concrete with plastic tarps to prevent evaporation. Just like being wrapped in plastic at a spa, the enclosure will create humidity that will continuously hydrate the surface of the concrete.
Lift the tarps once daily during the curing process. Spray the top of the concrete lightly with water using a garden hose. Replace the tarps.
Remove the tarps and cease spraying the concrete after the recommended curing time for your project. Remove the formwork as well, once you are sure the edges are strong enough.