I always order a few extra bunches of leather leaf fern. It is invaluable for filling in open gaps in the design for a very reasonable price.
Leather leaf fills in open spots quickly.
Fan it out behind other greenery for a full rich look.
I typically finish the backs of my church florals so they can be transported to the reception hall for "double duty". This way if they have to stand on an open table, the back still looks finished.
Don't go overboard, but fill in enough that you don't see the florist foam.
Hiding the mechanics is the mark of a professional looking design when you are finished.