This section is from the "Nature Cure: Philosophy and Practice Based on the Unity of Disease and Cure" book, by Henry Lindlahr.
The effectiveness of breathing exercises and of all other kinds of corrective movements depends upon the mental attitude during the time of practice. Each motion should be accompanied by the conscious effort to make it produce a certain result. Much more can be accomplished with mental concentration, by keeping your mind on what you are doing, than by performing the exercises in an aimless, indifferent way.
Keep in the open air as much as possible and at all events sleep with windows open.
If your occupation is sedentary, take all opportunities for walking out of doors that present themselves. While walking, breathe regularly and deeply, filling the lungs to their fullest capacity and also expelling as much air as possible at each exhalation. Undue strain should, of course, be avoided. This applies to all breathing exercises.
Do not breathe through the mouth. Nature intends that the outer air shall reach the lungs by way of the nose, whose membranes are lined with fine hairs in order to sift the air and to prevent foreign particles, dust and dirt, from irritating the mucous linings of the air tract and entering the delicate structures of the lungs. Also, the air is warmed before it reaches the lungs by its passage through the nose.
Let the exhalations take about double the time of the inhalations. This will be further explained in connection with rhythmical breathing.
Do not hold the breath between inhalations. Though frequently recommended by teachers of certain methods of breath culture, this practice is more harmful than beneficial.