Don't underestimate the importance of warming up. Spend about five to 10 minutes doing some kind of aerobic exercise or a light exercise that utilizes the same muscles you'll be working on. This will get your blood flowing and, if it's a good warm-up, you should begin to sweat. Warming up should not be confused with stretching. They serve two separate functions.
Some trainers recommend stretching before the workout, and some after.
Either is fine as long as you begin the session by warming up. Concentrate your stretching on the body parts you'll be working on to help prevent injury and reduce the probability that you'll have sore muscles after the workout or the next day. The duration of the stretch should be between 10 and 30 seconds, and you may want to repeat the stretch two or three times. Extend until you feel definite stretching, but not so far that it's painful. This will stretch your shoulder and triceps.