The traditional tossing of the bridal bouquet goes all the way back to medieval Europe and stems (please excuse the pun) from the fact that the bride was said to be especially lucky on her wedding day.
After the wedding ceremony was over, the bride was essentially attacked by guests who attempted to rip off pieces of her dress to take home to increase their own good luck. (So much for preserving that bridal gown for future generations!)
In order to avoid the post-ceremony riots, brides began tossing their stockings (hence the garter custom) to the ravenous crowds.
Finally the tradition settled on tossing the bridal bouquet which was a more lady-like option for brides.