I've lifted for a long time and would always try to go as heavy as possible, but after reading Mentzer's book I realized that I had gotten out of touch with REALLY feeling the weight. If you watch people in the gym, 99% of them are just going through the motion. Mentzer's basic theory is that you should lift enough weight that 6 reps is failure. The weight should be lifted at 4 seconds up, 2 second hold, and 4 seconds down (add that up and it's precisely 60 seconds). People always talk about the last few reps of a set being the most important, but what does that make the initial 9 reps in a 12 rep set? Cardio? The idea here is to make ALL your reps as hard as those last two reps and only do 2 sets of 6 for each exercise.
I'm very happy following this thus far, I'm stronger than I've ever been.