Wow. A whole lot of great responses.
I think brides should know two things.
One is that this is something they've never done before (plan a wedding) and they need not only all the help they can get, but the drive to hang in through this intense planning time.
Second, if I could wave a magic wand, I would wish for them to go to the future. Go into the future and have their future selves tell them what they need to know now.