Hygiene in the deepest sense is about purity of mental and physical action - does our activity include much, or little, unnecessary effort? How graceful is our action? How integrated and powerful is our mind?
The Hygiene of Effort could be seen in successive orders, we can see how our effort can be more or less pure in each of the five orders of conditionality - physical, biological, psychological, ethical and existential. Most of us, when we think of hygiene will usually just think of the second of these five, and even then only perhaps in the superficial sense of washing our biological self.
Ultimately, we can aim to reduce all unnecessary effort. Having a mind which is completely pure of unnecessary effort is an aspect of full enlightenment. At the same time, it's important to know that effort is only necessary in relation to what our goal is, and has no fixed or inherent nature.
In the picture above Monkey fights with Princess Iron Fan. Monkey was rather naughty and has been tamed by the Buddha, to put his effort into good use. Click on the picture to get its Wikipedia page for more info.