Metabolic rate, not to be confused with metabolism, is the rate at which your body burns calories or the speed of your metabolism. Both metabolism and metabolic rate are affected by many factors such as age, gender, lifestyle, and hormonal levels. The thyroid gland produces hormones which regulate how fast or how slow your body burns calories and for such things as when your body uses energy to build muscle tissue from protein or stores energy as fat.
The most obvious modifiable factor for affecting your metabolic rate is lifestyle (e.g., physical activity and diet). Exercise, including weight training and cardio, will elevate the body's basal metabolic rate (BMR), which is essentially a baseline metabolic level, even during rest. Weight training, in particular, elevates one's metabolic rate because it increases muscle mass and muscle is a very metabolically-active tissue within the body. As the body ages, muscle mass slowly decreases and as a result, so does metabolic rate. A double whammy is the gain in bodyfat along with the loss of muscle mass, compounding the problem. Interval training (e.g., high-intensity interval training known as HIIT) is a very effective technique used to stimulate positive changes in one's BMR.
Nutrition also plays a part in terms of affecting metabolic rate. Not eating enough food (less than 1200 kcals daily) slows down metabolic rate because the body "thinks" it's starving. Going too long between meals (e.g., more than three hours) will slow down metabolic rate. On the other hand, a relatively high-protein diet will boost metabolic rate because protein is a macronutrient which the body must work harder and therefore expend more calories to digest. Caffeine and some spicy foods (e.g., hot peppers) can boost metabolic rate due to their stimulant qualities.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."