The most obvious way to identify a flower is by color and flower shape and, in many cases, that is enough to make a positive identification. I generally take pictures of flowers while I am hiking and identify them based on the pictures once I get home so I don't have to carry a heavy field guide with me. I struggled for a long time to identify flowers of similar shape and color because I wasn't taking a picture of the leaves as well. The leaves can be just as important as the flower itself in identification.
Fall brings three great examples of the importance of looking beyond the flower itself and considering the whole plant. The flowers of Ageratina altissima (White Snakeroot), Eupatorium perfoliatum (Boneset), and Eupatorium hyssopifolium (Hyssop-leaf Boneset) look almost identical. They all bloom in late summer or early fall, so season is not a good indicator of their identity, either. Here are pictures of the flowers for each plant (click to enlarge):
To tell them apart, one has to look at the leaves (shown in the same order as the flowers):
petiole, between the leaf and the stem of the plant; and they are about half as wide as they are long. It is found in woods and thickets.