I love using a mix of different greenery. This gives texture and height to the overall arrangement. Myrtle has long lines that look great in very large florals.
The leaves are broad and glossy green. Set some aside for your bouquet and corsage work.
Choose a long lateral for your center insertion that will the the tallest in your arrangement.
Place it straight up and down in the center of your floral.
Angle out other (slightly shorter) pieces from the center out.
The beautiful contrast of greenery will make your final design look lush and full.