This Container Gardening Web site by Alabama Extension is one of the most helpful I've seen on container gardening. It includes guidance in selecting appropriate-sized containers for various plants, choosing plant combinations for containers, and caring for your container plantings. The file size is very large so be patient with the download.
Clever Gardening Technique
Make a tomato-watering device to provide your tomatoes with a good soaking that wets the soil deeply. Punch a dozen holes in a milk jug or gallon can. Set it beside the plants soon after planting and firm a little soil around the base of the container to prevent water from escaping quickly. Put a few stones in the gallon jug or can for weight to prevent it from blowing over on windy days. Fill the container with water and it will slowly leak out into the soil rather than running off like water from a hose would. Add a tablespoon of fertilizer to the container prior to filling for an extra boost.