Buy healthy, green plants with no signs of wilting or disease. Plants should be stocky, with plenty of leaves. It's actually a plus if they don't have any flowers on them - they'll divert their early energy into root development rather than flowering.
Choose a site in full sun, although zinnias usually appreciate a little afternoon shade in especially hot regions. Zinnias like rich, well-drained soil.
Plant seedlings or seeds in spring, after all danger of frost has passed. Seeds can be sown directly in the soil, 1/4 inch deep and a few inches apart. Thin to 6 to 12 inches, depending on the variety's mature height. Seedlings, which will produce earlier blooms, should also be spaced 6 to 12 inches apart, again depending on their mature height.
Keep soil somewhat moist. Mulching is a good idea. Zinnias can survive in soil that's on the dry side but will wilt in very dry conditions.
Trim faded flowers, or cut flowers often for bouquets, to promote more and longer blooming.
Fertilize every four to six weeks, or work in a slow-release fertilizer (or plenty of compost) at planting time.
Tear out and discard plants in fall, after frost fells them.