You'll add to your portfolio as you practice and experiment with new finishing techniques. You may not realize that some faux finishes have become somewhat of a commodity. For example, sponge-painted walls and faux marble have fallen into the commodity category because anyone who enrolls in a class at his local crafts store or continuing education program can learn how to do them. There's still a market for professionally applied basic marble finishes, and mastering marbleing techniques helps build confidence. Then you can move into various metallics, textures, European aged processes, and very decorative designs, which currently are the hot finishes because they aren't so common.
Photographs are good portfolio presentation tools, but actual samples are better.
Make sure that your samples are large enough to give prospects a feeling of the finish's color and texture. Think more along the lines of poster boards than paint chips when preparing your portfolio. Decorative painting jobs tend to be for upscale clients and often lead to other more traditional jobs. The book, in addition to containing beautiful photographs of myriad finishes, offers a complete section on the business of decorative painting. Catalanotto suggests using a simple flier instead of a fancy brochure to get the word out to your current customers that you now do faux finishes.