Choosing a florist begins before you ever enter the florist's shop. Take a moment and consider your first impression. Remember, you are hiring a florist professional for their attention to detail and their ability to create an atmosphere that invites you in. If their shop doesn't invite you in or seems extreme in relation to your personal style, most probably the wedding flowers they provide you won't be satisfactory to you either. There are always exceptions, but the chances are that this rule will prove true in your case. Do you really want to gamble on your wedding flowers just because your cousin Freda insisted that you had to use this particular florist?
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.
Though it can be difficult to do, try not to get your heart too set on a particular type of wedding flowers (the possible exception to this might be your basic red rose). Flowers are beautiful but being living things they can be temperamental. Some flowers, such as gardenias and lilies are famous for triggering allergic reactions. If you have someone in your wedding party with bad allergies it might be wise to steer clear of flowers like these. Likewise, hollow stemmed flowers, such as the daffodils can drip sap on your wedding dress. Yes, this is a problem that can be overcome, but you need to be aware and prepared so your wedding flowers don't ruin your wedding dress. Other flowers, most notably the lilies will drop a yellow pollen stain on your gown if you don't have the wedding flower professional remove the stamens before you carry them down the aisle.
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.