Do at least 15-20 minutes of cardio before stretching your muscles-stretching cold muscle can cause injury; also do your stretches slowly and no sudden twisting motion, especially with your back. I've had surgery a few times from sports related injuries and did learn a few things from my physio.
Good job on the recommendation to participate cardio work before stretching. However, the jury is still out on the amount of time needed to adequately elevate the body's core temperature before stretching. There are many factors that will affect this 'preparation' phase; time of day, age of participant, etc. In general, spend a minimum of 5-10 minutes participating in low intensity cardio activity to increase the body temperature and blood flow to the muscles intending to be stretched. Remember to stretch the muscle being used in the workout. For example, a runner, would spend significant time stretching lower body muscles (i.e. calves, hamstrings, etc). Holding a stretch (static) for between 15-30 seconds has also proven effective. Stretches done during the warm ups should be held for a shorter time frame than those done at the end of a workout.
Keeping these additional tips should not only help keep you flexible, but also reduce your risk of injury.