Single, larger blossom flowers, such as roses, carnations, zinnias, tulips or lilies, usually make up the focal points of a piece. Taller stems with multiple flowers can balance things nicely. Delphinium, gladiolus, and snapdragon are good examples of flowers that can add extra height and texture. "Filler" flowers, or stems with many small buds, are great for topping of a bouquet with a full and delicate look.
When I learned Ikebana as an adult, I was really fortunate to have had an old_fashioned sensei (instructor) who not only taught me the theory and techniques, but also immersed me in the respectable manners and ways of the good old days. It is through my sensei that I learned that Ikebana goes beyond the typical art form, and that it is more of a way of life; in fact, Ikebana is also known as kado, which literally means "the way of the flower."