A good brass player spends many hours in front of a mirror watching and listening. A good rule to follow is to form the embouchure so that all the muscles around the aperture are working in unison. Picture a fireman's net - the center being the aperture of your embouchure. Around this net are eight firemen pulling out from the center to keep the net taut. Think of your muscles working in this same way to preserve the stability and openness of the aperture. Keep a close eye on the area just below the bottom lip, the goal being to have as little movement as possible and to maintain a flat chin. In addition, try to form your embouchure away from the mouthpiece and the instrument. A good brass player knows how to do this.