The one fact that you can do that will always help you get the best haircut is... Shampooing your hair before your haircut...
Clean Hair Is Just Better
Yup clean hair.. it actually cuts better, cleaner hair allows the clipper to do their job better. Dead skin cells, dust and debris can clog the teeth of the clippers resulting in a more "dull looking" cut. Your barber must be (1.) conscious of this and (2.) brush the teeth of the clippers more frequently throughout the haircut to keep teeth free of debris. Clean hair of course looks cleaner, shines and appears more moist and healthy (all of which will reflect in your haircut).