I am a professional graphic designer and the number one problem area is communication. It is the graphic artist’s responsibility to ask as many questions as is necessary to gain a complete visualization of what the client actually wants the final product to look like. This will lead to reversions, complications and nothing but frustration for both people.
BE SURE to have an idea of what you want before you search for a designer. It doesn't need to be an exact description, but make sure to you have a general idea, and the designer should figure out the rest through a short interview. (A larger project might require a lengthy interview, as well as follow-up interviews.) Remember that artists understand "feelings" more than any other terms. Describing a "blue" cover for a book means potentially 4 million different colors, but saying that you want a "cold scary feeling" will limit the choices greatly.
Often times, the client will tell us that we have "freedom" in certain aspects and this almost always leads to complications. I've learned many times in the past that the client’s idea of "freedom" seldom reflects the artist's interpretation of "freedom." Anytime a client gives me any "freedom" I usually take this as that they haven't thought about it enough and I'll ask questions until I get enough information to figure out what they want. Even if they don't like it later, they can't say that's not what they wanted because they are the ones that said it.
Also, be sure to find an artist that has a portfolio and pricing policy that suits you. After you hire him ask for comps. Comps are speculative pre-work composite layouts that the artist will use to design the finished product. It might cost a little more at first but it will ensure that you have a better idea of the finished product. Some artists will do free comp work if the project is big enough and you've signed a contract stating that you will hire them for the work.
Beware of artists that do comps for free without contracts. These artists typically do not uphold the ethical considerations of graphic design. They are usually desperate for work (which should tell you something) but also will cut any and all corners to get the job done. They typically work with the approach that quantity over quality. I once had a client that used one of these types and got sued. His website not only used copywritten images without consent of the holder, but also used design elements from another site. He not only had to hire a lawyer to fight that and pay damages to the other company, but also had to hire me to remake his website. Again, it's not worth it.
One final note, look at the artist's portfolio and ask for references. Take into consideration that the bigger the project, the larger the list of references. Also, a good graphic designer will let you talk more than he/she does and typically, if you are not talking, the designer will be asking questions.
NOTE: I've owned a graphic design company for 9 years and all this information is based on my experience. However, I might be wrong.