Show the florist the wedding flower pictures and the swatches of your wedding dress material (and any other prominent materials from your wedding i.e. bridesmaids dresses). Talk. Make sure the florist grasps the concept that you have in mind for your wedding flowers and understand your budget and any other constraints that you are operating under (i.e. facility rules, etc.), and then let them go to work. Resist the urge to micromanage. In most cases at least one of the florists you visit can make your ideas for wedding flower combinations come to life in a manner that you never dreamed of. At the same time, however, go over every detail of the proposal (including set up time and schedule - which will vary greatly depending on how much there is to set up for your wedding flowers and how many other weddings the florist has that same day) carefully before you sign the agreement to provide your wedding flowers. Get several proposals for your wedding flower needs before you decide. It's your wedding so be sure that you are getting the best wedding flowers for your budget.

Two last notes about your wedding flowers. First, remember that not all flowers fare equally well in all types of weather. Some flowers wilt quickly in warm weather while other flowers actually turn black in colder temperatures. Either way you won't be happy with the flower's performance if you take it into a situation for which it is not suited.
Finally, remember that despite the advances in horticulture around the world, flowers (even wedding flowers) are still seasonal. Sure, if your pockets are deep enough, you might be able to have the florist import them from the other side of the globe but that will be expensive and even at that, there are some flowers that just won't be available to join your wedding flowers in celebrating your marriage unless it's just the right season somewhere.