All 50 of the U.S. states have a Power of Attorney form that you can use to give one or more person(s) the right to act on your behalf, which you can limit to one specific activity, situation, and/or event in the future. Also, if you wanted you could empower someone to act on your behalf for a variety of tasks.
A Power of Attorney can be either immediately effective or it can be exercised exclusively on the occurrence of an event in the future. In the Power of Attorney you may specify whether the right to act on your behalf is temporary, continuous or permanent.
If later on you decide you want to revoke the Power of Attorney given to the declared person(s) you can do so. You will need to file a form called a Revocation of Power of Attorney form. Some states may require a "revocation copy" along with your Revocation of Power of Attorney form. A revocation copy is a copy of your original Power of attorney form with "REVOKE" written in large letters at the center top and then signed plus dated.
You may be wondering why it might be necessary to give anyone else the power to act on your behalf. Consider this, what if you were buying assets and you couldn't be there that day, your declared Power of Attorney could make that legal transaction for you on your behalf. A Power of Attorney is not simply for convenience though. What if you were incapable of acting on your own behalf because of an illness, unconsciousness, or because of an accident; your Power of Attorney could make those vital, life-saving decisions, and have the directive along with the legal authority to inform the medical staff of your healthcare wishes. Some people get more specifically what is called a Healthcare Power of Attorney form that empowers someone to act on your behalf just for a medical situation/event.
In some situations where you are unable to manage your own personal or business affairs, and you don't have a Power of Attorney, then the court may appoint someone to act for you. You will not have a say as to who maybe appointed. However, if you have a Power of Attorney the right to choose a person to act on your behalf and the authority given to them is totally up to you.
If you have not yet filled out a Power of Attorney form, it is about time you started thinking about getting one. It will remain valid as soon as you sign it and it will stay valid until you die or when you revoke it.