It happens to all of us. Young / old, weak / strong, shielded / open. Eventually we all run smack dab into a system overload of energy that we cannot use or is not our type. It leaves you feeling out of sorts, not like yourself at all. Often you end up acting out the physical manifestation of that energy leaving you feeling trapped inside your own body doing things that you would not otherwise do.
For me being a negative feeder, I need to resist the urge to wear bright floral shirts and hum along to the greatest hits on the easy listening station. For most others it is negative energy that overwhelms and they see themselves acting out in a decaying cascade of aggressive actions that they feel powerless to stop. In all cases it not only affects you physically it does so mentally. It leaves you unclear, unfocused and feeling utterly helpless until the non-useful energy works its way from your systems. Learning to ground and center effectively can help purge a non-useful energy overload and help return you to a balanced state more rapidly.
The American Heritage Dictionary defines to meditate as "To train, calm, or empty the mind, often by achieving an altered state, as by focusing on a single object". And understand that meditation is only part of the answer. In grounding & centering you use the empty mind to drive the physical state as well and in doing so burn off the non-useful energy from your system. It is that singular centered focus in both the mental and physical that makes meditation a useful tool.
Now before you get all glassy eyed and start looking for a guru, understand that there are many types of meditations. While the classic vision of sitting cross legged and chanting may have its place for focus, it is not the most useful tool for burning unwanted energies expediently. Moving mediations that involve a great deal of physical movement are actually the best. And the thing is you probably already do some of this by instinct. But defining it as grounding & centering will help you channel and direct some of that which you already know.
A moving or physical meditation is one that involves rote, repetitious physical action that body knows by instinct so well that it allows the mind to detach. Doing so allows both the mentally and physical selves to attain a centered clarity void of any intent or purpose, save that of the action itself. You become the physical activity, the energy, and in doing so are able rapidly to dispel it from your system. In sports this is sometimes referred to as being in "the zone". In essence you, the activity, the energy become one. It is that synthesis, the emptiness, that clarity of mind, body and energy that allows one to rapidly purge their system and return to a state of balance and control.
Simple physical activities are best. You want to engage the physical in a demanding way, yet be able to let go and detach. Softer, rote more mundane actives seem to be best for purging energy that is more positive. While negative energy is most effectively purged in a hard and fast manner.
But in both cases, it is completely dependent on the individual.
For me a mundane "soft" activity allows me to find and locate that unwanted more positive energy. I use the repetitious action involved in such things a housework (ironing, vacuuming, dishwashing, folding laundry, etc), cycling, walking, and driving as a means of reaching and obtaining my desired zone. For expelling negative energies, I suggest something harder and faster. Running (not jogging), throwing rocks, hitting a heavy bag, whacking a bucket of balls at the driving range, heavy yard work, to name a few. These are just suggestions. There are many more hundreds of such activities, each one personal to the individual that can be used as just an effective means. The important thing is to reach that centered state of being – of oneness – so that you can ground out that unwanted energy.
Once that state of being is achieved, you will begin feeling a bit better, a bit more like yourself. Do not do the obvious thing and stop the activity. Remember your goal. You are purging your system of unwanted energies. You need to keep pushing until you begin to feel tired in order for this to be an effective ground. Do not push yourself to the point of physical collapse, but do push to the point where you feel a comfortable tiredness. You will actually feel the letting go. Once that happens you will need to "close" the activity and return to your normal balanced state. When I say to close and activity I mean to give it a formal end. Doing so allows you that few moments of formal detachment to regroup, re-shield and regain your balance. Once have come back to your balanced self, if you still feel a bit exhausted, you may need to feed lightly on energies best suited to your needs. If you feed only do so at a surface level. You want you give yourself time to rest, renew and recover before returning to your normal feeding patterns.