The effectiveness of this exercise depends to a large extent on how well you execute it. My advice is to spread your hands slightly wider than your shoulders (at the lowest point of the press-up your forearms should be at right angles to the rest of your arm).
Hold your body weight on the way down, lowering yourself gradually and deliberately, and then push yourself back up with an energetic movement.
You can perform the exercise with your hands and feet on the ground, with your feet elevated, or with your hands elevated, and with your hands spaced apart as above or close together.
One variant of this exercise, called the pike press-up, allows you to work out your shoulders: Place your hands on the floor and stand on your toes; raise your buttocks as high as possible while facing the floor. Bend your arms to lower your body, head to the floor, then push yourself back up to your starting position. This exercise works out your front shoulders.
Do two to five sets of 10 to 30 reps of each exercise, with a 30 to 60 seconds rest between each set.