How to avoid doing things that suck the energy out of you?
This is one of the things you learn from a live teacher, not usually found in books. Briefly, few of the main points I was taught, in no particular order:


ego - a lot of energy is spent on defending the self-image, rationalizing, finding excuses etc.
negative/pessimistic thinking - another major energy hog is thinking of the kind "things are wrong and are impossible to fix", excessive focusing on the issues and bad news.
masturbation and sexual activity - orgasm is traditionally considered a major point of energy loss.
non-spartan lifestyle - relaxing into one's weakness for sensuous comfort.
having no agenda - allowing other people, things, and circumstances drive you around.
inner conflict - blaming or judging oneself non-constructively.
doubt - anxious wavering back and forth.
not taking responsibility - organizing one's action around other people's approval and acceptance instead of taking ownership of one's choices.
alcohol - consumption of alcoholic beverages.
low energy food - food that is spoiled, expired, frozen and reheated, over-processed, lifeless, and generally bad quality.
posture - an important factor of energy loss is a posture and gait that moves the body weight off the center of the feet and too much towards the toes or the heels.


I will add more points here if I remember any.