Heavy Days: Once a week, pick a certain body part or muscle group and go to the maximum with a power move to work that specific muscle group. For example: when training legs, max out on squats. For chest, max out on bench press and so on. By doing this, your body will be taxed to the extent that it will not recuperate before your next workout. This will help you to develop your strength and gain an accurate perception of your progress.
Stretching: Muscle, tendon, ligament and joints are naturally flexible, but this means that they can also stiffen and limit your range of movement. However, they can also stretch, giving you a wider range of movement as well the ability to contract additional muscle fiber. Stretching before you workout actually allows you to workout harder. By stretching the body parts you are going to workout, your body will adjust to the heavy resistance of the weight, thus making it possible to workout longer and without fear of suffering damage to stiff muscles.
Breathing: It is very important that you do not hold your breath while working out. If you hold your breath, you can most definitely injure yourself along with depleting your strength. Breathe out with effort while you are exerting yourself. For example: When doing squats, take in a breath as you stand with the weight on your shoulders and squat. Then expel the breath as you push yourself back up.