Energy, the Energy Parami. If we look at the Energy Parami, the most important energy that we need to develop is mental energy. There are different ways to develop mental energy in the practice. One way to develop mental energy is to have reflection type meditations that stimulate a sense of urgency. These include the reflection on death and the reflection on Dukkha.
Confidence inspiring objects of meditation can also stimulate energy. Developing the compassionate motivation is one of the best ways to stimulate the Energy Parami, and that entails opening to Dukkha.
So the most important way to develop energy is to develop mental energy because understanding the nature of human beings, we don't always have the useful zest of physical energy. As we age it is quite important to know how to bring forth the mental energy despite the absence of strong physical energy. One thing that helps bring about physical energy despite the aging process is a sense of determination to not allow the body to be identified so much as being oneself. Not taking it personally, so that we can see the changing nature of the particular sensations in the body and know how to practice despite the lowering of the physical energy.
So mental energy is very important and knowing ways of reflection that can stimulate this. How fortunate we are to have the opportunity to practice, realizing that we may lose the opportunity at any time. Developing compassion for ourselves and others by understanding the nature of our opportunity, of meeting with the Dhamma, so that we can bring up the energy to make good Kamma. Reflecting on a sense of refuge in the Noble Eightfold Path and joyful objects can help, too, to inspire energy.