Flowers are certainly the most traditional and lavish the tops of frosted cakes. There are different types of flowers used: those made with Royal Icing, Buttercream or Gumpaste, Fresh Flowers, Crystallized Edible Flowers and already-made Silk Flowers. 

Royal Icing dries hard and is used for many flowers such as daisies, roses, violets, pansies, etc. Royal Icing is edible and is candy-like.

Flowers can be made by piping Buttercream into roses or flowers with or without leaves. 

Gumpaste is best used when you want very hard flowers and other decorative items. It is a sugar dough that is rolled out and shaped into elegant flowers such as (but are not limited to) lilies and orchids. These flowers are the most realistic and are often made on toothpicks or floral-wire in flower arrangements. Gum paste arrangements are especially stunning cake toppers that can be kept as a keepsake for years to come. What's more, you can do them in advance, let them air dry, and keep in an open container, in a cool, dry place, until you place them on the cake. Although these flowers are especially beautiful, you should plan on paying extra for them due to the time it takes to make each gum paste flower. 

Fresh Flowers: The culinary use of flowers dates back thousands of years to the Chinese, Greek and Romans. Flowers have traditionally been used in many types of cooking: European, Asian, East Indian, Victorian English, and Middle Eastern. Today, there is a renewed interest in edible flowers for their taste, color, and fragrance. 

Some flowers can be stuffed, such as squash blossoms and then fried or used in stir-fry dishes. Edible flowers can be crystallized; frozen in ice cubes and added to beverages; made into jellies and jams; used to make teas or wines; minced and added to cheese spreads, herbal butters, pancakes, crepes, and waffles. Many flowers can be used to make vinegars for cooking, marinades, or dressings for salad. Herbal flowers normally have the same flavor as their leaves, with the exceptions of chamomile and lavender blossoms, where the flavor is usually more subtle.

Edible flowers can be used fresh as a garnish or as an integral part of a dish, such as a salad. When decorating a wedding cake some will make a large corsage type arrangement on top and/or sprinkle some petals around the edge of the cake platter for a lovely finishing touch. 

You can decorate with crystallized edible flowers (and/or fruit), made in advance, or use fresh edible flowers. Fresh flowers are also very popular and beautiful, but some flowers can be dangerous if ingested. Please take caution when choosing the types of flowers in the arrangements for the cake. Use those that haven't been sprayed with pesticides (or gather some from your yard that are organic). (NOTE: Lily of the Valley and others are toxic. List of Safe and Edible Flowers). 

There are different ways to decorate with edible flowers: Make large corsage type arrangements which are easy to place on the cake when delivered.  Another way is to clip greenery and flowers directly before the wedding and place on table surrounding cake and on top. Or, sprinkle some petals around the edge of the cake platter for a lovely finishing touch when the cake is served. 

Place flowers directly on the cake. To keep them fresh by the time you serve the cake, cut a 3" piece from a drinking straw. Bend one end up and tape it. Then fill the straw 3/4 full with water and stick the flower stem in. Arrange the flowers on the cake. Wilton, a cake decorating resource, makes already made flower holders that are hidden in the cake.

Most florists I have worked with for wedding cakes would make arrangements designed to be placed in the space between the cake tiers. So if I had a cake with a 7-inch space between the tiers and say about 12-inch round they would take those dimensions and put together a small arrangement that fit into the space. It would usually be a plastic cup with the green "frog" material inside and water. Then the flowers would cover the container and I would just have to set the arrangement in the tier while setting up the cake. This way the flowers never touch the cake.

Using fresh flowers also means that you MUST get together with the florist and discuss delivery times. It isn't feasible with delivering cakes to place flowers on before serving when you are having a reception. A cake is usually set up the morning of the wedding, even if the reception is for the evening. If I am doing a cake with fresh flowers I will schedule the time later, about the time of the actual wedding so the cake is set up as close to the reception time as possible and the flowers MUST be fresh and cold when setting up the cake. Most reputable florists are accustomed with the needs of fresh flowers on the cake.