Keeping proper form is important in all exercises. Here’s a great article and link to a site to help keep your form proper.
Proper Bench Press form starts lying on a Bench with your feet on the floor. Unrack the bar with straight arms. Lower it to your mid-chest. Press it back up until you’ve locked your elbows. Keep your butt on the bench. Bench Press sets of five reps every StrongLifts 5×5 workout A.
The Bench Press is a full body, compound exercise. It works your chest, shoulders and triceps most. It’s the most effective exercise to gain upper-body strength and muscle mass because it’s the upper-body exercise you’ll lift most weight on (more than Overhead Press). The bigger your bench, the bigger your chest.
To avoid shoulder pain, tuck your elbows 75° when you lower the bar. Don’t try to stretch your chest by flaring your elbows 90° out. You’ll impinge your shoulders if your upper-arms are perpendicular to your torso at the bottom. Tuck your elbows 75° to Bench Press pain-free.
Unlike the Squat or Deadlift, the bar doesn’t move in a vertical line when you Bench Press with proper form. It moves diagonally from your mid-chest over your shoulders. This is the safest way to Bench Press for your shoulders. It’s also the most effective way to Bench Press heavy.
This is the definitive guide to proper form on the Bench Press.