The Italians are really good at this. And at this time of year it's such a nice thing to do. The button hole often remains empty but I'm a mission to fill it. The flower you choose should be picked yourself, don't get a flouncy shop bought variety. Even a common-a-garden daisy or dandelion (if a bit sticky) would do. Style it in, don't go crazy "look at me" Oscar Wilde styleé. Choose something subtle, pretty, delicate, it may only last a couple of hours, but that's the point. I've chosen here a beautifully soft mauve auricula - the Victorians were crazy about them. The flower should be your only decoration, so no patterns or prints. I chose a plain Dior Homme white shirt and black cashmere Aquascutum Ltd. jacket. I tried a deep purple hellebore last week. n.b. didn't last long.
If you put the stem in boiling water it makes it last longer. I picked mine at 10.00 a.m. yesterday and it was still going strong at 1.30p.m. when it fell off in the street. But that's the joy, the fleeting moment.