The ghost ball method for pool requires you to imagine the cue ball's position at impact along the line of centers--the cue ball pinned on the optimum line through the object ball that drives the target ball to the pocket. Most pool pros do not consciously use this method of aim!
Many, not all, pool pros instead aim directly for the contact point on the object ball--despite the geometric fact that dictates that the ghost ball method is the correct line of aim and that contact point aim will bring a miss.
The pro either subconsciously makes any needed adjustment to aim while firing right at the contact point or else lines up on the ghost ball aim line but CONSISTENTLY eyes and gauges the contact point.
A contact point hit allows for a typical more full hit on the object ball, for enhanced power and control. Additionally, the pros can hit more thick than most amateurs because they hit more softly than most amateurs, and thus "throw the balls in." (Yes, I know that some pros SPIN the balls in using outside english also, but that is a discussion for another day.) Softer hits will generate more throw at impact.
The amateur billiards player instead struggles to shoot toward the ghost ball. That difficulty is compounded by trying to aim at an imagined point in space. A fine pool player is always targeting at something they can see instead by using the actual ball as a visual target.
Further, most amateurs shoot more thinly then they aim. When aiming at the ideal ghost ball, they tend to overcut the object ball for a miss. When reorienting for the contact point or even more thick, they tend to cut more thinly and hit the ghost ball contact point.
Advanced pool players who know geometry will avow that the contact point is so thick on non-straight in shots that balls will be driven straight into rails. But if you try my method and do plow balls straight ahead into the cushions, you already a) pocket balls like a professional with both your stroke and aim exactly where you think you aim (you can stand, see and shoot better than most) b) you're already shooting softly, too.
I'll rephrase that last paragraph this way--if you miss many shots, try my method by hitting the balls more thick and more softly than before--for most readers, far more softly. If you are already play at a semi-pro level or above, forget this entire article and keep doing what you're already doing.
So here's how to do it the pro way and teach yourself any needed aim compensations over time, also:
1. Locate the target point on the object ball, the furthest possible point on the ball from the center of the pocket drop. Some call this spot "the back of the ball" (relative to its intended pocket).
2. Walk back behind the cue ball to find the place where a line from the target point would split the cue ball in half through its absolute center. This spot must be obeyed and will often yield a more thick (and yes, sometimes thinner) cut shot than instinct tells you to aim at--that is, if you've been struggling to make shots before now. Players thus often find themselves in their physical stance with their feet far to one side of where they'd stand "naturally".
3. Shoot the ball. Score!
I welcome your questions. If you've explored the many instructional and entertainment articles at this Pool and Billiards GuideSite at About.com, you know I thrive on reader questions and teaching the marvelous sport of pool and billiards. Looking forward!