There should be an arch formed by your lower back, but make sure not to force it too much at the risk of injuring yourself. With this technique, the chest is ensured to take the full load of the weight and the shoulders don’t try to compensate.
When performing flyers, don’t fully complete the movement; stop the motion just before the end. Once you hit the top and bring the weights together, the pectorals are effectively given time to rest and are no longer under tension. For maximum efficiency, it’s key to leave the muscles under tension as much as possible.
And the most often forgotten but maybe the most important: work on your triceps! The triceps often aid the chest in many exercises, so by working on them, you can up the amount of weight you can lift when doing chest exercises. So don’t neglect them!