The boutonniere is to the groom what the bouquet is to the bride, and since in general there's just a wee bit less room for creativity in groom/groomsmen attire, it's a chance to bring in a little embellishment.
Martha shares some ideas. . .

This boutonniere is made from crepe tissue paper, for you
DIY-types

Dahlia boutonniere and polka dot tie

Nerines with hypericum berries and miniature cymbidium orchids with jasmine vine

This groom's orchid boutonniere is set off by a tiny Swarovski crystal frog. For reals.

Another one for the crafties: a ribbon boutonniere