It's all in the accuracy of the hit on the rack, and it helps to have something definite to aim at. For example, I shoot from the side rail and like to aim directly at the center ball in the rack (where the 8 would be in an 8-Ball rack). Some players break farther from the side rail, and they probably aim at some other target in the rack. The important thing is that it's definite and pinpoint, like the center or edge of a ball.
The rest is just consistent inside spin and easy natural roll speed to stop the cue ball where you want it on the long rail, probably a little above the second diamond.