The problem most people have hitting off the rail is that they put slight unintended english on the ball and it causes the cue ball to curve a little before it hits the object ball. This may cause the illusion of poor aim because if you are aimed corectly, but the cue ball curves, you will nearly always miss (and if you have a somewhat consistant stroke, you will miss in the same direction).
There are several ways to help make the ball in this position.
1) Hit DEAD center. Easier said than done, but probably the best way.
2) Hit harder. Hitting harder reduces the curve in the shot, thus making you slightly more accurate. The only problems are that many people stroke less accurately when hitting harder (especially when jacked up) and that you will sacrifice some degree of cue ball control with this method.
3) Angle your cue more parallel to the table bed. A more parallel cue reduces any curve you might put on the ball to some degree. This may seem difficult to do because you are jacked up on the rail, but if you experiment a little, you will likely find that you can hit the ball more with a more level cue than what is intuitive, especially with softer speeds.
4) Shoot while accounting for the curve. If you put english on the ball purposefully and just account for the deflection and curve, there are plently of instances where it is easier to make the ball rather than just straight center. This takes quite a bit of experimentation and feel to pull off effectively, but helps out often.
The shots I have trouble making are the medium difficulty shots that follow a great recovery shot or a series of tough shots. In other words, I tend to underestimate shots when they don't seem challenging.
