When using large calla lilies for bouquets or centerpieces, you'll have to deal with the natural fiberous nature of the flower. The flowers often have a curve and bend that you may have to manipulate to straighten.
Professional florists sometimes roll the stems in layers of newspaper or poster board, leaving the stems exposed at the bottom and putting in water.
Never stand the stems in more than a couple inches of water, since too much soaking can soften the stems and turn them slimy and introduce bacteria into the stems.
You can knead the stems gently, breaking the fibers with your fingers. Pull the stems gently through your hands, helping to straighten the curve.
Work with each stem individually until you are satisfied with the stems.