The key for me when it comes to developing a greater focus on the target muscle group, that I think a lot of people overlook, is figuring out what muscle is getting the greatest stretch at the bottom of the movement.
It's not uncommon to see guys complain that their shoulders take over their bench press, or their triceps, and then they upload a video and you can immediately see why. If your hands are in too close and your triceps are getting a good stretch at the bottom of your bench, they are going to get quite a bit of stimulation.
If you aren't moving your shoulders or arms correctly, or not holding your chest up, or bringing the bar down to the wrong place, it's very easy to get a stretch in the front deltoids that immediately wakes them up to take a brunt of the work.
You have to remember that the whole point of bodybuilding exercises is to fully stretch and contract muscles... just because you're doing a bench press doesn't mean your body knows it is supposed to be building your pecs. It's going to give the greatest stimulation to whatever muscle is getting the fullest stretch and contraction during the movement.
This is the same reason you don't fully lock out your elbows when doing pull ups or barbell rows... because then you are just asking your biceps to get more involved.
This is also the reason why I don't believe much in things like pre-exhausting a muscle, because you haven't resolved the real issue.
Long post for some pretty simple advice: tweak your form in the bench press so that you are stretching (not excessively) your pectorals at the bottom of the movement, and not your shoulders.
How would you stretch out your pecs without the barbell, just using a pole or a doorway? Or with no assistance, just swinging your arms? Try to imitate that feeling under the bar. In a bodybuilding bench press, the shoulders move back to make way for the chest as the bar descends, so that most of the stretch and squeeze is on the pecs.
You don't want to try and get some crazy stretch and rip your pec off the fucking bone, but you do need to tell your body that the chest muscles are the ones taking it through this movement.