Q: You constantly warn against using too much weight, but aren't heavy weights necessary to develop big muscles?
A: The most formidable challenge in bodybuilding is developing the ability to tell the difference between "too much" weight and "not enough" weight. There is no objective measure of what's too heavy or too light, and even our subjective determinations can deceive us. Having said that, there are ways to figure out if your target muscle for any given workout is being attacked with the correct amount of weight to stimulate new growth.
Here are some procedures for testing the effectiveness, or fatigue level, of the most important exercises for each bodypart. The best method to gauge whether a target muscle has been trained to the limit is to follow a core exercise with a movement that isolates the target muscle.
Note that none of this "testing" is quantitative, in that I'm not suggesting a limit for the maximum weight you put on a bar or on a low-rep limit for each set (those vary, depending upon the bodypart and the exercise). My only concern is that you find the ideal weight/rep combination that allows you to optimally fatigue the target muscle in the shortest time.
THE RONNIE COLEMAN TEST
This is the classic compound exercise for legs, but it's also the most resistant to cheating: No matter how many other muscle groups you try to incorporate into good old-fashioned barbell squats, your thigh muscles are going to get the best workout. Nonetheless, you should test those muscles by following your last set of barbell squats with bodyweight squats - no weight on your shoulders, but going below parallel. If you cannot do more than 10 reps of these, then you had a good squat workout; otherwise, use more weight for squats.
When you've finished with these, try seated cable rows with the weight you would normally use for 10-12 reps. If you cannot do more than six reps of strict seated rows, then you had a good barbell-row workout. If you can do more than six, you need to increase your poundages for barbell rows.
Test the effectiveness of your flat-bench presses by going to cable crossovers without rest - as if supersetting. If you can do more than half of your normal reps, then you need to pay a penalty: Return to the bench press for two more sets, this time going to serious failure.
After your last set, go immediately to dumbbell lateral raises. Grab your heaviest weight. If you max out at three strict reps, you get a star pasted on each delt. If you can do more than three reps, then it's back to the drawing board for more militaries.
If you think you've gone to failure on these, move immediately, as if supersetting, to a preacher bench and try another very strict set with the same weight. If you can do only two reps, you pass.
Lying triceps extensions
To make sure you got everything possible out of your lying triceps extensions (cambered bar or barbell), rest for your normal interval, then try another set with the same weight, this time upright and extending above your head. You should not be able to do more than two reps, and even those should be a struggle.