Workouts, like any activity in life, are best when planned and executed well. Just walking into the gym and doing whatever you feel like might satisfy you in the moment, but the long term benefits are miniscule at best. Instead, approach each training opportunity with a plan that has a defined beginning, middle and end, and understand that the decisions you make in the midst of your exercises will dictate how well your body responds to the demands you impose upon it.
The Do: Some people skip stretching, and others who elect to do it often do it wrong. So, let's clear up the stretching myth once and for all. Yes, you should stretch before and after all workouts. However, stretching cold muscles at the beginning is actually worse than not stretching at all. Rather than jumping into a wide lunge or a deep squat, take five minutes to get your body moving and warm. Jog briefly on a treadmill, run in place, ride the stationary bike, or jump on the elliptical. Doing so lets your muscles loosen up, which readies them to stretch properly. If you do this, you'll find that your stretching is far more productive and your flexibility will increase drastically over time. Once you have finished your training, dedicate some time for a cool down stretch. This engages the muscles, shakes up any acid build-up and allows the body to transition as well.
The Don't: During the middle portion of your workout, the part that requires the most intensity and exertion, you must consistently remind yourself to not become a slave to numbers. Great training reflects pushing the body to its limits, which often means near failure. Doing so will maximizes your muscles' potential. If you maintain a certain number you want to reach, you potentially limit yourself. Stopping on ten, because that was you goal, when you could do fifteen sells the workout short. Instead, push to your maximum and know that the last three need to be hard. If they're not, you need to either do more reps or lift heavier weights.