The very basic principle that any personal trainer worth his or her salt will tell you: Begin your workout with the large muscle groups and work your way down to the smallest. Why? The smaller muscles usually work to link the larger muscles, and, if you exhaust the smaller muscles first, it's very difficult to get the larger groups working to their capacity.
When carrying out an exercise, a lot of people think that the most important part of the movement is getting to the 'top'. In fact, the lowering, or the 'negative' action of an exercise is often more important than the lifting, or 'positive' aspect.
For example, you should take more time lowering yourself into a chair than you do lifting yourself out of it. A rule of thumb is that you should double the amount of time lowering a weight as you did lifting it.
The rate of muscular development (shaping) is directly proportional to the effort that's put into an exercise.
To be effective, an exercise must be completed to the point of momentary muscular failure.
Performing an exercise to this level will ensure that the muscle groups being exercised have been worked to their maximum potential, ie. during a single set of exercises, you should try to build up the intensity to the point where you cannot complete another repetition without losing form.
At this point, you will have reached your maximum intensity, and you should then take a 30-60 second rest before continuing with your program.
It's imperative that you maintain the exact form of each exercise. To that end, you must try to perform each activity slowly and precisely through its full range of movement. Each repetition should mimic the previous as closely as possible.
If you start to lose form, stop and rest or (if applicable) reduce the amount of added weight you're using. A badly executed exercise can be both ineffective and cause injuries.
Would you get into your car on a cold winter morning and immediately red-line it without warming it up first? No. Well the same is true for your body. One of the best ways to avoid injury is to warm up before you exercise and warm down afterwards. 'Warming up' is just that - bringing the temperature of your muscle fibers, tendons and ligaments up to working temperature, and preparing your cardiovascular system for the forthcoming workout.
Cooling down is as important as warming up, but it's often ignored. Immediately after an intense workout, you need to keep the blood flowing in order to flush the lactic acid from your muscles.
A light program of stretching will assure that this action is carried out. Spend 5-10 minutes both warming up and cooling down. Light aerobic exercise followed by a series of stretches will do the trick.