2. The Principle of Overload
The principle of overload states that a greater than normal stress or load on the body is required for training adaptation to take place. The body will adapt to this stimulus. Once the body has adapted then a different stimulus is required to continue the change. In order for a muscle (including the heart) to increase strength, it must be gradually stressed by working against a load greater than it is used to. To increase endurance, muscles must work for a longer period of time than they are used to. If this stress is removed or decreased there will be a decrease in that particular component of fitness. A normal amount of exercise will maintain the current fitness level.
3. The Principle of Progression
The principle of progression implies that there is an optimal level of overload that should be achieved, and an optimal timeframe for this overload to occur. Overload should not be increased too slowly or improvement is unlikely. Overload that is increased too rapidly will result in injury or muscle damage. Exercising above the target zone is counterproductive and can be dangerous. For example, the weekend athlete who exercises vigorously only on weekends does not exercise often enough, and so violates the principle of progression.
The Principle of Progression also makes us realize the need for proper rest and recovery. Continual stress on the body and constant overload with result in exhaustion and injury. You should not (and can not) train hard all the time. Doing so will lead to overtraining and a great deal of physical and psychological damage will result.
4. The Principle of Adaptation
Adaptation is the way the body 'programs' muscles to remember particular activities, movements or skills. By repeating that skill or activity, the body adapts to the stress and the skill becomes easier to perform. Adaptation explains why a beginning exercisers are often sore after starting a new routine, but after doing the same exercise for weeks and months the athlete has little, if any, muscle soreness. This also explains the need to vary the routine and continue to apply the Overload Principle if continued improvement is desired.
The levels of adaptation are quite complex. A more detailed explanation can be read here.
5. The Principle of Use/Disuse
The Principle of Use/Disuse implies that you "use it or lose it." This simply means that your muscles hypertrophy with use and atrophy with disuse. It is important to find a balance between stress and rest. There must be periods of low intensity between periods of high intensity to allow for recovery. The periods of lower intensity training, or the rest phase, is a prime time for a bit of crosstraining.
6. The Principle of Specificity
The Specificity Principle simply states that training must go from highly general training to highly specific training. The principle of Specificity also implies that to become better at a particular exercise or skill, you must perform that exercise or skill. To be a good cyclist, you must cycle. The point to take away is that a runner should train by running and a swimmer should train by swimming.
While there may be other 'principles' of training you will find on the web and in text books, these 6 are the cornerstone of all other effective training methods. These cover all aspects of a solid foundation of athletic training. Once put together, the most logical training program involves a periodized approach which cycles the intensity and training objectives. The training must be specific not only to your sport, but to your individual abilities (tolerance to training stress, recoverability, outside obligations, etc). You must increase the training loads over time (allowing some workouts to be less intense than others) and you must train often enough not only to keep a detraining effect from happening, but to also force an adaptation.