Some tips:
Make sure all trim matches. If you go black, go ALL BLACK. If you like carbon fiber, go all carbon. Silver door handles and a gunmetal windshield surround would clash. No unity.
If the front of the car looks OEM, insure that the rest of the car does, as well. Canards, fairings, and vents sticking out of the back of an otherwise "stock" body give the car a confused look. Are we getting groceries, or race trophies?
Make sure the mod does not DECREASE the functional ability of the car. If you increase rim size, keep it light and upgrade the brakes. Otherwise, you'll accelerate wear and tear on the chassis.
Have a theme in mind. A goal, from start to finish, is key in producing a magazine-quality car. Brabus rims combined with Lorinser interior accents combined with a Carlsson front spoiler = confused ride. Each tuner focuses on a different direction in modification... Keep brands together. Mutts never sell for a high price for a reason... they offend all the purists.