I worked in a mantle shop for about 6 months a few years ago. My first suggestion is to check local building codes on clearances, materials, etc. You can build a spectacular fireplace surround but if the customer runs afoul of his insurance company because it doesn't meet code, it could put you out of business very quickly. My second suggestion is to make sure your design is well balanced to its surroundings. If you can find books at the library on mantle building it would be a good idea to read them.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.