If some deck movement isn't a concern to you, you can use precast concrete blocks that sit on top of the soil. These work best on flat ground and when the deck isn't attached directly to your house. Look for blocks that have slots cast in the surface where the floor joists can sit. Place sand or gravel under the blocks, both of which react well to seasonal temperature changes and promote drainage to help prevent erosion. Sand and gravel allow water to seep and flow away from the footings. Be sure to use enough concrete blocks to distribute the weight of your deck. Too few, and they'll sink into the ground.
Below Grade Foundations
Below-grade foundations are the best, most secure way to anchor a deck in place, no matter where the deck's placed. You'll need to know how deep the frost line is in your area. Consult with your local building department or county's agricultural extension office. Other factors to consider are the seepage and flow of water around the foundation. A cone-shaped foundation allows for somewhat better water movement than a cylinder or square footing. Most local building codes dictate that decks attached to homes and other structures must be supported by below-grade foundations.
Poured Concrete Foundations
Many deck builders choose to pour concrete directly into foundation molds. A popular type of mold is round cardboard tubes. To use them, dig a hole with an auger or post hole digger, true and secure the cardboard form in the hole so it's perfectly upright, then fill the tube with concrete. There several different types of tubes available, including some that include square or round plastic bases that also fill with concrete. These may be required by building codes because the bases help keep the foundation from sinking into the soil, and the concrete can extend above the ground to help keep the bottom of the wood posts dry to prevent rot.
An alternative to creating your own foundations out of cardboard tubes and plastic bases is to purchase precast footings from a home or builder's supply center. This will save you time because you won't have to pour the concrete and wait for it to set. This will probably cost more than using molds, however. Even with an added cost, precast footings have become a popular alternative to pouring foundations, since you save multiple steps. They come in a variety of shapes, including cylinders, cones, and cylinders with bases.