1) Breathing correctly during a workout can dramatically affect your performance. When I am going through an exercise, easy or challenging, I take a long inhale(five seconds in), hold it in for three seconds, and then exhale(five seconds out). 

2) Every repetition should be slow and in control; think of doing a bicep curl for 5 seconds up to the curled position and 5 seconds down to return to the relaxed position.

3) When in doubt, do another repetition. Too often, we stop at a predetermined number, although we could do more.

4) Stretch as often as possible to prevent injury and build strength. Do a set of an exercise and follow-up with a stretch.

5) If you ever feel discomfort or pain, stop the exercise to stretch.. If the pain persists, stop the exercise altogether.