Get your body ready to exercise.
Always warm up before you work out. Warming up properly (i.e., doing enough movement to raise your body temperature) helps to ensure that your body is prepared to safely handle the stress of physical exercise.
Don't put yourself at risk.
Don't exercise if you're ill or feeling unduly fatigued. Physical activity can exacerbate your illness if you're sick, and possibly expose you to an increased risk of being injured if you're extremely tired.
Organize Your Workout Environment.
Keep your exercise area free of clutter. Rerack weights. Rehang towels. Don't allow your exercise area to become a makeshift obstacle course, which can elevate your odds of being injured (e.g., tripping, falling, etc).
Know how to perform the various exercises that comprise your workout regimen. If you use equipment, adhere to the manufacturer's instructions for using each device.
Have a game plan.
Establish an accurate benchmark for your starting level of fitness, and then, based on that assessment, develop a sound (i.e., safe, efficient, and effective) plan for achieving your fitness goals. Be cognizant of the fact that your plan is a dynamic entity. As such, put it in writing and periodically revisit it to determine if any adjustments to it are appropriate.
Never try to do too much too soon. When you first begin to work out, start at a sensible level, and then progressively adjust how much you do as your body adapts to the demands imposed on it.
Use your lungs.
Never hold your breath while exercising, particularly when you're lifting weights. Holding your breath while exercising can cause a buildup of inner thoracic (inner rib cage) pressure, which can inhibit the return of blood to your heart. This action also can compromise the amount of blood flow (and subsequently oxygen) to your entire body.
Don't ignore what your body may be trying to tell you if you're doing too much when you're exercising. Always respond to such signals (e.g., sharp pain, dizziness, nausea, etc). Having a "warrior" mindset will not protect you if you exceed your body's capability to adapt to the amount of physical stress imposed on it.
Don't let the forces overwhelm you.
Always control the speed of the weights you're lifting while strength training. Dropping or throwing the weight can cause an excessive level of load force being placed on your body's skeletal joints. The same adage applies to your body. Keep the amount of bouncing and explosive movements to a minimum.
Know what to do if you need help. If you exercise in a health/fitness club, be aware of the facility's emergency plan and where key safety-related items are located (e.g., first aid equipment, antiepileptic drugs, etc). In addition, know the location of the nearest telephone for getting assistance and the contact number for obtaining it. Whenever possible, work out with either an exercise partner or a personal trainer.