There are some basic principles that govern the world of exercise, and knowing them can help you set up and manipulate different components of your workout.
The F.I.T.T. Principle
FITT is an easy way to remember the exercise variables you can manipulate to avoid boredom and to keep your body challenged:
Frequency - how often you exercise
Intensity - how hard you exercise
Time - how long you exercise
Type - the type of exercise you're doing (e.g., running, walking, etc.)
When you workout at sufficient intensity, time and frequency, your body will improve (also called the Training Effect) and you'll start to see changes in your weight, body fat percentage, cardio endurance and strength. When your body adjusts to your current FITT levels, it's time to manipulate one of more of them.
For example, if you've been walking 3 times a week for 20 minutes and you've stopped seeing improvement, you could change your program by implementing one or more of the following ideas:
Frequency - Add one more day of walking
Intensity - Add short bursts of jogging, speedwalking or hill training
Time - Add 10-15 minutes to your usual workout time
Type - Do a different activity
such as cycling, swimming or aerobics
Changing any of these variables every 4 to 6 weeks can help you keep that training effect going.
Progressive Resistance (the Overload Principle)
In order to improve your strength, endurance and fitness, you have to progressively increase the frequency, intensity and time of your workouts. A simple way to stimulate your body is to try different activities. If you normally walk on the treadmill, try riding the bike which will use different muscles and allow you to burn more calories. If you've been doing biceps curls with dumbbells, change to a barbell.
Specificity
This principle is just how it sounds...how you exercise should be specific to your goals. If you're trying to improve your racing times, you should focus on speed workouts. If your main goal is simply health, fitness and weight loss, you should focus on total body strength, cardio and a healthy diet. Make sure your training matches your goals.
Rest and Recovery
While we often focus on getting in as much exercise as possible, rest and recovery is also essential for reaching your weight loss and fitness goals. While you can often do cardio every day (though you may want to rest after very intense workouts), you should have at least a day of rest between strength training workouts. Make sure you don't work the same muscles two days in a row to give your body the time it needs to rest and recover.