Understanding the principles of what we're doing helps us make logical decisions in creating a great routine.
When lifting weights or using weight machines we are tearing tissue so it can heal stronger and with more mass. There are two parts to this equation: the number of sets and the number of repetitions (reps). Repetitions are the number of times we repeat an exercise in a row and sets are how many times we repeat a group of repetitions.
A general guideline is: 3 x Sets, Set 1: 12 x Reps, Set 2: 12 x Reps, Set 3: 10 x Reps
It is advisable to do 3 sets of an exercise and gradually increase weight each set while doing 12 reps each. In the final set, aim to fatigue the muscle before the 10th rep. If the muscle is not fatigued by the end of the third set then it's time to up the weight.
So, that will tear the tissue, now it's time for the growth. Giving the muscles you have worked time to heal is critical. Stretching after a workout helps with tension and pain. Give each muscle group one or two days of rest before working it again.
Form is everything. This is where the expertise of a trainer or an experienced workout buddy can come in handy. Learning the proper techniques for handling the equipment will insure that you get the most out of each rep and that you prevent injury.
There is a fine balance between working to fatigue and hurting yourself. Be aware of what your body is telling you and if unsure it is better to back off than push too hard.
Lift with your knees! You do not want to throw your back out.
Empower your core. Your core (abs and lower back) is the place where all your energy starts. A strong core will help you in all other areas. Make sure your exercise routine includes plenty of core strengthening exercises.
Remember, not every day is going to be perfect. There will be days when you just can't get into your zone. That's okay, do a lighter workout but don't quit. Packing your gear the night before, having a partner to motivate you, and being in a regular gym routine can all help on these off days.