I have heard a number of suggestions andideas about how to train, but there are a number of principles that can't be disputed. Generally speaking, over-training varies from individual to individual depending on genetics--fast and slow twitch muscle fibers. Look for the signs of over-training. Briefly, they are: not sleeping well, muscle soreness stretching out to several days, lack of progress, irritability, etc. Also, workout routines should vary and choice of exercise does make a difference. Use a variety of techniques and be aware that there are certain exercises that are contra-indicative (they invite injury or should never be done or one should learn proper technique).
One last point, the amount of time you spend in the gym does make a difference. Research does indicate that your level of testosterone during a workout does make a difference in what kind of results you can expect. For most men, testosterone peaks in the first hour and then begins to diminish thereafter. It has been concluded that if you get your exercise in during this period of time, you will see the best results. So, working out more than hour and a half is not very productive for almost everyone.