There is much more to bench pressing than just lying on a bench and moving a weight up and down. If you don't care about getting stronger, or prefer to injure yourself while bench pressing, then sure, go ahead and simply lay down and start pressing. But if you would like to bench press safely and effectively, you need to learn to set up on the bench correctly.
I come across many benchers who use too relaxed of a set up. The incorrect images in this post could be described as "casual", "lazy", or "slack". We want to have a "deliberate", "active", or "tight" set up. You may get by with a lazy set up for a while, but if you want to keep adding weight to the bar, you will need to put more effort into setting up.
It should feel like work before even un-racking the weight. You will be pushing your feet into the ground and your back into the bench. Your entire body will be tensed up. If you feel like you could stay in position for longer than the duration of your set, (usually 30-45 seconds) you aren't set very well.
Tension and Leg Drive
Most people consider the bench press to train the chest, shoulders, and triceps. While it is true that these muscles are the prime movers, there are many more muscle groups involved during the exercise.
A good bench presser makes the bench press a full body lift. They are able to produce muscular tension throughout their entire body. This is what allows force to be transferred from the ground - through their body - and into the implement being moved. The first step to generating tension is by producing leg drive.
To create leg drive, simply push your feet into the ground and contract your glutes (as if you were going to push your hips up and off the bench). Once the hips are set, the torso can tighten, from which the arms can press off of. This must be maintained throughout the entire set. If you allow your feet to slip or lift off the ground, you have lost leg drive. This alone takes some practice and conditioning.
Think of bumper boats. When two boats collide, the force dissipates, and both objects move away from each other. When we are lifting weights, we want to be able to move a load with our body staying anchored in place.
If you aren't tight enough, the force you are attempting to apply to the weight will dissipate throughout your slack body parts. Good leg drive is the first link in the chain to creating and maintaining total body tension.
A good indication that you aren't creating sufficient leg drive is that your feet are placed directly below or in front of your knees. They should be planted on the ground back toward your shoulders. Your knees should be bent at an angle less than 90 degrees.
The Back
The back plays a major role in any pressing movement. The back muscles will properly align the spine, set the shoulders, and help control the load eccentrically. In the case of any pressing done on a bench, the back muscles will also create a firm base to dig into the bench.
When setting up, your back shouldn't be relaxed. Pull your shoulder blades back and down (retract and depress). Arch your upper back as you do when puffing out your chest. Throughout the set you will be driving your tight back into the bench.
Left: Chest is puffed out, shoulders are set back, and the angle of my knees suggests I am creating leg drive. Right: Chest flat, shoulders not set, no leg drive.
I want you to remember what I said at the beginning of this article...a good set up should feel efforted. If you could just as easily grab your iPhone and flip through Instagram, you're not prepared to make bench pressing gains!