One of the first things that you need to do before breaking a rack in pool is to select the right type of rack. Selecting the right type of rack would play a very significant role in determining the quality of break, which is why you need to carefully make the choice. A simple rule of thumb for racks is that the tighter it is, the better the break will be.
While playing the shot, make sure you put enough power into it. A common mistake made by amateur pool players is that they try to produce this force by striking the cue by with a lightning-fast shank. Shooting faster is not the most appropriate and effective way of putting power and consequently force into the shot. The proper way of putting power into the shot is by pulling the stick farther and then sliding it forward in a smooth motion to make a steady and square impact with the cue ball. Make sure to complete the follow-through. Playing a shot this way would put extra power into it.
The position from which you take the shot is really important. Instead of resting your stick on the back rail and breaking from there, get as close to the rack as possible, i.e. the head-string. Form a hand-bridge and rest your stick on it to take the shot.
Instead of simply striking the cue ball with the stick, learn to strike it in the right area. The best place to make the hit is just below the middle of the cue ball. Striking it there would allow you to put plenty of power into the shot, while at the same time allow you to make the ball die after it makes contact with the other balls. This will put you in a better position for your next shot.
Instead of aiming for the second row of balls, go for a full hit on the head ball. This would most likely result in a stronger and more consistent break.