What makes a training program successful is the combination of proper techinque with an effective training schedule. All too often, I have observed many individuals working out in an aimless, random manner -- they simply train on the basis of how they "feel" on a particular day, without following a systematic Training Program. Indeed, these individuals may workout on a regular, consistent basis -- weeks, months and years on end -- yet have little to show in the way of meaningful progress.
One of the main reasons for their lack of progress, I believe, is that they do not have enough structure to their workouts: in other words, they are not following a Training Program.
The benefits of following Training Programs are numerous. It allows you to organize your workouts, to create better balance between antagonistic muscle groups (i.e biceps versus triceps), ensuring symmetrical development of the body from head to toe. For instance, many people tend to favor training those body that respond well to exercise, whilst neglecting those parts that are more stubborn -- as a result, over time their bodies will develop imbalances, leading to a less pleasing aesthetic result.
Another benefit of following a Training Program, is that it helps to ensure proper recovery of your muscles between workouts. Those individuals who train in a haphazard manner often fall into the trap of training their muscles before they have optimally recovered from the prior workout -- this can lead to over-training syndrome, and also possible injuries over the long-term.
Following a Training Program also fosters variety in your workouts! The human body will typically respond beneficially to a new training program for a number of months, but will eventually plateau, and results may stagnate. At this point, an effective Personal Trainer will alter your training program -- manipulating set, reps, and selection of exercises -- to surprise and stimulate your body into new adaptations, and therefore continue progressing.