Machine Shoulder Presses target the front and side deltoids. Because a machine is fixed, the movement becomes extremely strict and you are able to feel resistance from a full extension to a full contraction at the top. Consider Machine Pressing to be a shaping and finishing exercise instead of one designed to build mass.
Hold the grips of the shoulder machine at shoulder height and push up till your arms are locked out. Now, allow the weight down under control to the starting position, getting a maximum stretch at the bottom. You can also perform a behind the neck variation of the shoulder press.