Before I got a personal trainer I used to spend 5 minutes stretching before I started my gym workouts. I was told that was the worst possible thing to do!
Apparently, your muscles must be warmed up *before* you stretch. An workout should always *end* with a comprehensive stretching routine (around 15 minutes). Moreover, you should stretch affected muscles in between sets and individual exercises to stop them shortening. Muscle fibre shortens (but increases in diameter) when it is exercised.
Do any of the gym members on here do enough stretching?