There are probably many methods available to develop the natural Qi (breath / energy) flows within our bodies, but one has only heard of how the Chinese develop Qi and acquire more from nature through martial arts like Taijiquan, Qigong, and Neigong, which involve various levels of breath control. And the backward flow meditation which according to Lu Dongbin was known as the Circulation of the Light during the times of Laozi and Guan Yinshi (c 550 BC).
From discussions with a few Taijiquan experts, a Qigong expert, some practitioners of advanced meditation, and from personal experience, it appears that the backward flow meditation could be the fastest method to develop and to acquire additional Qi. While the martial artist(s) may practise the art a decade or more before he or she can feel the Qi flows, a practitioner of the backward flow meditation may feel the flowing Qi within a few years of practice, when both are at rest (that is not during or immediately after their respective practices).
For those practitioners and martial artists who wish to strengthen their existing Qi, if they feel it is weak, they can try these ‘methods’ (obtained through my own contemplation) which one has found to be useful:
Think about your Qi.
If you still cannot feel the Qi flows, think about it at nine (9) p.m. your local time.
The Qi flows will appear stronger at certain times of the day. Note the particular times down for your own study and understanding.
If you read an article on Qi in the web or elsewhere and the Qi starts to flow spontaneously, then your Qi is strong and flowing 24/7. If your eyelids feel heavy and cannot seem to lift up because of the Qi flows, then the Qi is mature. If too mature, the Qi will dissipate and be wasted. Perhaps you will then have to learn how to deal with the mature Qi, if you have not already done so.
After further intensive training, perhaps your Qi will travel?