The Straw Experiment
Sit with reasonably good posture. Breath normally through the nose and spend some time becoming aware of your breathing without trying to change it or achieve anything. Have an ordinary drinking straw ready in the hand. After the start of an average exhalation put the straw to the lips and allow the air to pass through the straw instead of the nose. Take the straw out just befoer the end and allow the last bit of air to pass out the nose. Do not do this for every breath. After each 'pass' with the straw, allow the breath to adjust in whatever way it will.
Rationale: This is a way to naturally prolong exhalation, which will also naturally decrease the breaths per minute. The resistence posed by the straw also increase the 'back-pressure' which helps to keep the airways open and increase oxygen exchange. Placing the straw after the start helps with avoiding gulping air in in anticipation of having to 'blow it out. Removing it before the end helps avoid forcing the last bit of air out unnaturally. There is a technique known as pursed lip breathing which has some of the same goals, but when the lips are pursed it tightens many muscles reflexively that are freer when the straw is used. Not using the straw each time avoids over-adapting to an artificial situation. The benefit comes from triggering reflexes and a possible 're-synching' of body rythyms.
Source: Carola Spreads