Centering, Grounding and Shielding were some of the first skills I learned when I started down this path. They form such a basic core of my "being" that I scarcely give them much thought anymore, the pathways are so ingrained, that a simple signal is needed and they are in force.
Before we can begin, I think we need to learn and recognize how to use our own personal energy. This can be done in a simple manner.
Find a quiet spot and sit down and relax. Take a few deep breaths. I find it helps to start with a simple phrase, chant or even a count to ten. If you do this every time you want to "center yourself" you will find that just by repeating this phrase you will begin to be able to relax with out thinking about it.
The next thing to do is to determine the boundaries of your energy or aura. Bring your hands together as if you were going to clap them, but very slowly, at a certain point you should feel a subtle resistance. This is your aura, don't be discouraged if you don't feel it at first, Take a deep breath and try it again. Feel confidant about this before going on to the next step.
Once you feel comfortable and can recognize what this feels like, try visualizing or feeling more energy coming out of your hands to push them further apart. Then try shrinking it by drawing that energy back into your hands.
You will probably find that one hand is better at "pulling" and the other better at "pushing".
Now practice the pulling with one hand and pushing with the other. Imagine an energy circuit flowing out one hand into the other, letting it go down to your solar plexus, and out again through your hand, make the circuit many times, until you feel you have mastered it.
There are times when we are stressed or have been to a ritual that we feel "all charged up" sometimes it is enjoyable and sometimes it's time to get rid of some of this excess so that we can calm down.
Remember Which of your hands is your "pushing hand" . Now that you have mastered the flow of energy out from this hand, you may use it drain off some of the energy. Some folks find it helpful to carry a stone with them (hematite is good) that will absorb your energy. When it "gets full" I find its best to get rid of it and get another, though some folks will cleanse them for use anew. You can also touch the floor or ground ( I actually find its helpful to slap a wall or floor) and visualize or feel the energy "going to ground"
Another way is to visualize/feel it draining out through your feet, kind of like a bathtub draining, this works too.
Again, I find it helpful to have a short phrase that you use every time, this signals your brain, "lays in the circuit"so to speak and makes it easier to do over time.
Have you ever had someone "violate your personal space"? Your brain sent you a signal to move back. This is a form of the shield almost everyone carries around them without even knowing it.
Now think back to the hand exercise. Remember how your aura felt? And how you were able to increase it? This aura covers your entire body. It is your "early warning system" . You can practice recognizing this and strengthening it by closing your eyes and having some try to touch you.
Practice doing this until you can "feel" them before they actually touch you.
Some folks imagine a brick wall or actual armor all around them. Personally I find this too heavy to carry around!! I visualize a "polarized (like the sunglasses) egg shape all around me. It filters and reflects back any unwanted energy, emotions or just yucky day to day stuff. It lets in all the nice stuff.
Every night before I go to sleep I extend it out over my house to protect my loved ones.
Some folks find they are empathic and easily affected by outside emotions or energies, all the more reason to practice your shielding untill you feel it's as hard as diamonds.
Practice is the key here. Make a concerted effort do do this every day, until it comes with no effort. It takes a while, but soon it will be something you don't have to think about, it will aways be there for you.