Controlling your personal (spiritual) energy is actually pretty simple. As yogis and zen masters can tell you, it takes a lot of practice to get really good at it, but what doesn't? No matter how good or bad you are at controlling your energy, you can always get better, so there is absolutely no reason not to start practicing this today (especially if you are concerned about evil presences around you.)
Some emotions come from subconscious responses to stimuli, but most of our emotions can be controlled with our thoughts. In fact, even the subconscious responses can be trained over time using this same technique. Just as a karate master knows how to block a punch before he has time to think about it, your subconscious can be trained to automatically respond in a certain way. That comes later, of course, but it isn't really any different from what we're talking about here - controlling your thoughts is the key to controlling your energy.
So how do you control your thoughts? It starts with awareness. Start becoming very aware of your responses to certain things. What makes you smile? What makes you angry? Does somebody do something that instantly annoys your or sets you off? What are you afraid of? Start noticing your emotional response and try to put it into words. Why do you feel this way? What logic is your mind using to justify this emotion? Challenge it. Most of the time if we dig deep enough we can see that our fear, anger, anxiety, and even depression is unjustified and blown out of proportion.
Being aware of these irrational thoughts and fears is the first and most crucial step to getting rid of them. They can be replaced with positive thoughts and responses just by "rewiring" your logic and connecting the stimulating events with either positive or neutral responses. It's surprisingly simple, yet far too often ignored.
So back to the demons... Demons feed off of negative energy, so if you get rid of the negative energy the demon has less power, less time in our plane, and thus less effect on people and things that exist in our plane. Understanding this is VITAL to keeping evil away. Energy attracts like energy, so if you are putting out positive vibrations, you will not only be attracting positive vibrations, but you'll also be repelling negative ones!