Many of my brides ask about flowers for their hair. A large bloom tucked behind the ear or pinned in your hair looks gorgeous. You could wear this in lieu of a veil or underneath the veil but either case - it's a small detail that people notice. The problem is that there aren't that many flowers that can withstand the hairspray and oils that comes with a pretty hairdo. You also need a flower that's light enough to be pinned and that can look fresh for a long period of time.
So here a couple of good options....
Cons: Sometimes the petals will turn brown over time and it has a strong smell. Some like it the smell and others don't.
If you like the look of the flower but don't want to deal with the scent then a silk flower would be a good option. It seems like there are a lot of varieties of hair clips with gardenias.
Cymbidium Orchid (Comes in white, pink and green. Other colors a little harder to come by would be yellow and orange)
Vanda Orchid (orange, fushcia, purple)
Stephanotis Bloom (white)
Phalaenopsis Orchid (white and purple)
Cons: These also tend to wilt sooner rather than later. You would probably have to pin a new one in after the ceremony.
Dendrobium Orchids (white and fuschia and purple)
The nice thing is that pretty much any flower looks gorgeous in hair, it's just a matter of choosing one that fits your wedding style.