I like to make boutonnieres interesting by using a mixture of different foliage. One of my favorites for boutonnieres and corsages is plumosa.
This lacy fern has delicate fronds that look awesome in cut flower work.
You only need to snip one or two laterals to frame the back of your design. Don't be afraid to trim or snip off wild parts into a more manageable pieces.
Place the first frond behind the existing boutonniere and secure with florist tape.
Place a second piece in FRONT of the design. This helps give depth and interest when you surround the flower head rather that just always layering it on the back.
Continue adding plumosa until you are satisfied with the effect. Don't go overboard and do so much that you remove the focus from the flower itself.