How to maximize lung oxygen intake?
You cannot change the oxygen concentration of inhaled air in the absence of an external source of oxygen. However, you can use your lungs at their maximum capacity doing this:


breathe deeply: =increase tidal volume. Your muscles will inflate your lungs as much as they are able to, inflating parts of the lungs that were not fully inflated (atelectases) in the process. This also allows you to inhale a volume much greater than your physiologic dead space (volume of inhalation that does not participate in gas exchanges)
breathe to high enough frequency: the faster you will want to expel $CO_2$ and replace it with room air, the higher frequency you will need to adopt.


Frequency and tidal volume are the two components of minute ventilation (= (frequency/minute) X tidal volume), which is the physiologic value you are working to increase.

Position: The upright position is best suited, since the weight of the lung tissue pulling downwards will facilitate inflation

WARNING: using those techniques unsupervised can cause acute respiratory alkalosis, which is unhealthy and puts you in immediate danger.