The benefits of deep breathing are many, including stress management, clearing the mind or increasing concentration by improving oxygen supply to your brain. When it comes to exercise people often forget to breathe properly, often holding their breath while performing a number of repetitions or while exerting with resistance training. The general rule is to exhale during the hardest part of an exercise (when you exert the most pressure) and inhale at the easier part of the motion (when returning to the starting position) With certain exercises it may be more comfortable to inhale when exerting the most pressure i.e. with a seated row or lateral raise, your chest is expanding on the more intense part of the movement so inhaling may feel more comfortable. With cardiovascular exercise you may find it helpful to use the counts of steps while walking or jogging, or pedal revolutions on the bike to inhale and exhale i.e. inhale for four steps/revolutions, exhale for four to six steps/revolutions. Either way, maintaining some pattern for breathing while exercising is important to aid with oxygenating your muscles and avoiding feeling faint or shortness of breath.
An extra boost can be found by channeling your mind (and energy) to the muscles you are targeting using the breath in and out throughout an exercise. For example, as you inhale for the first part of the exercise (or for certain counts with cardio) imagine filling the body with positive energy and as you exhale through the second part of the motion imagine the energy being directed to the muscles you are working. This can supercharge your workouts by energising the muscles you are targeting.
For an added boost, include some outdoor activities to your regime to feed your system with healthy, fresh air or at least try to exercise with the windows open and as much fresh air circulating as possible.