I'll try to condense some of what I know into a lengthy post for everyone's benefit. This is based on my own personal experience. I am tired, so please forgive typos and such.
Ultimately, I would say that the more you are manifesting your true nature, the stronger your life force will be. There are many aspects of this. . The effects of this can range from simple physical manifestations, emotional manifestations, mental manifestations, and spiritual / etheric manifestations.
In the physical sense, a simple example would be running 5 miles and becoming tired, you have expended a good deal of energy.
In a Mental sense, an example would be judging yourself harshly, or thinking negatively of yourself or someone else. Thought is a creative force and this can create blockages in your energy body which prevent the life force from flowing correctly.
In the emotional sense, an example would be becoming angry at someone or getting in a verbal confrontation with someone, people who get in emotional fights are often draining energy from one another. A person who is being emotionally abusive towards another person is stealing energy from the person who is allowing themselves to be a victim in the situation. Anytime someone allows themselves to be a victim in any situation there is an energetic exchange that is happening, they are allowing someone to take their energy.
More can be learned about this process if you read about energetic vampirisim, and just study the mechanics of energy exchange between people in general. Energetic vampirisim is a practice in which individuals consciously learn how to drain other peoples life force. Individuals often perceive this need because their own life force is lacking. Many energetic vampires do this unconsciously, but there are other who do so consciously. If you have ever known a person who being around just made you Drained and tired, or even sick, this is an example of a person who is unconsciously or consciously draining energy.
Things get a little mucky when it comes to the ways that energy is exchanged on an Ethereal level. There are many energies and spirits which are parasitic in nature, they feed on other peoples life force. I spent a long time dealing with extremely intense & volatile energies that left me depleted and drained in the early phases of my shamanic practice. Most of the time they manifest through the mental sphere. They can also be channeled through other people or individuals. They are not "Wrong" or bad, as they are only being true to their nature, but it they are capable of causing great distress and tricking you into giving them much of your vital life force energy. They can manifest as voices, visually, an unexplainable presence, and in dreams, among many other forms.
The trick about all of these forms of energetic exchange, is that you can only be drained by another person or energy if you allow yourself to be a victim. This is especially true when it comes to Ethereal and spirit manifestations. You are the master of your own mind, and these forces can only drain your energy if you allow them to trick you into believing they have power over you. If you remember that you are a divine being of light, and stay grounded in your identity of truth, in union with Spirit, these things can not touch you. It is only when you forget your true nature that these forces have any power over you. Ultimately these forces manifest to teach us lessons about ourselves. They are teachers in their own way, to help us become stronger and more aware.
People have also developed many rituals to ward away energies like this (wearing a cross, getting crystals for protection, carrying certain herbs, practicing psychic shielding, the list goes on) - I used to use many of these methods but lately I feel like they are unnecessary if you are able to maintain a certain quality of awareness. To each their own.
depending on your degree of energetic sensitivity, your environment can also have a large effect on your energy levels. A messy and stagnant environment is likely going to make your own energy more chaotic and stagnant to some degree if you spend enough time in that environment, unless you are particularly strong willed. Where as a sacred space, a forest, etc can have a very empowering effect on your energy and life force. Our life force is strongly connected to the earth, and the stronger your own connection to the earth is easier it will be for you to channel that life-force energy. So spending a good amount of time in nature is a great way to increase your life force.
The food you eat also has a tremendous effect on the strength of your life force. There is a lot of evidence to suggest that a diet composed largely of simple and raw foods (nuts, fruits, vegetables) contains a very high vibration that will increase the strength of your life force. (do research before attempting a diet like this). I generally eat a diet of 50-75% raw foods, I avoid preservatives, corn syrup, msg, and sugar, as well as processed meats and dairy. This is a personal choice and different foods have different effects on people, but generally speaking processed food and refined sugars, flavor enhancers, etc, are all going create blockages in your energy body and prevent the creative force from flowing correctly.
There are many spiritual practices for cultivating and strengthening your life-force. Yoga, Qi Gong, Meditations, and some forms of energy healing are things that come to mind off the top of my head. Many of these practices strengthen the energy body and allow more energy to flow, while simultaneously helping to remove blockages that obstruct the natural flow of energy.
Also, just going to throw this one in for fun, electromagnetic frequencies and electronics that omit radiation can weaken your life-force from prolonged exposure. This is particularly true for cell phones and people often carry them with them for long periods of time.
These are just many examples, there are so many different aspects of this subject... Here is an article I wrote awhile ago that expands further on how being true to yourself and being true to your heart can greatly increase your energy levels, if you wish to read further.
http://www.evolver.net/user/cosmicdream/blog/power_surrender
There are also hundreds if not thousands of books on this subject, but I do not have any personal recommendations - I learned most of this directly from other people and personal observation/experience, which ultimately is the best way to do it - only you know if something is true. Take what resonates with you and ignore the rest.
I know nothing. Take everything I say with a grain of salt.
Blessings and Love to all beings.
Divine Manifest.
