Strengthen your rear delts, rhomboids, mid traps - ie: your scapular retractors.
Keep your shoulder blades squeezed together, use an athletic grip, and don't allow your upper arms to exceed a 45 degree angle with your torso (i.e.: your elbows should never be higher (in relation to your shoulders vs. waist) than your lower chest). Once your shoulder blades pull apart and your elbows begin to flair the side the emphasis is taken off the pecs and the front delts, triceps (and during some really horrible form), serratus/pec minor compensate.
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