If the actual sound seems much louder than the inner sound and drowns it out, crank up the volume in your head until your brain is full of sound. Now listen to the quality of the sound. You want the most beautiful, dark, rich tone possible. Add some vibrato and see how that intensifies the energy of the sound. If your mind wanders off to make distracting comments about the proceedings, gently but firmly bring it back.
Some people use a more physical approach.
Program yourself by mentally rehearsing away from the horn. Imagine picking up the instrument. Feel the weight of it and sense the wind blowing through the horn as you play great notes. If you have an upcoming performance, make it even more realistic by picturing yourself in the hall, the sounds of the audience, and the feel of your concert clothes. Now try playing the piece on your instrument.