Hard: Each exercise should be carried out to utter failure, where no additional movement is momentarily possible. You should find it necessary to sit down for a moment after finishing a heavy exercise. If you merely feel like sitting down, then the exercise wasn't hard enough. You should have to sit down to keep from falling down. Author Jones used to say "If you don't feel like throwing up after a set of barbell curls you haven't hit it hard enough.".
Breif: It is almost impossible to work TOO hard, but it is easily possible to work TOO much. In nearly all cases, if the intensity is high enough, one set of any exercise will produce better results than will two or more sets. A workout should not exceed a total of 45 minutes in length. 30 minutes per workout is a better goal. The number of exercises per workout should very from 10-20, depending on the equipment available.
Progressive: Always try to do one more repetition. When you can do 12 reps increase the resistance by 5% at the next workout.
Slow: It's very easy to perform a repetition to fast. It's difficult to perform one too slow. Reduce the speed of all your repeitions and you'll involve significantly more muscle fibers and radically decrease the probability of injuries.
Infrequent: Restrict your training sessions to not more than three whole-body workouts per week. Too often split routines lead to overtraining and a depleted recovery ability.
If you keep your rest between sets to no more than 0ne minute, with 30 seconds best, you can get 80% of your cardio in at the same time.