Whether you are signing a new power of attorney, or just want to revoke an old one, there are several things you should do in order to make sure that the old power of attorney is properly revoked.
If you are signing a new one, the new one will generally have language in it that states it revokes any old powers of attorney. However, you need to take some extra steps to ensure that the old one is not inadvertently used. If you have given copies of the old power of attorney to the agents appointed in that document, get the document back from them. If you cannot, send them a letter by certified mail informing them that the document has been revoked and any authority given to them or another person under that document is no longer valid.
If you had given a copy of the document to a bank or other financial institution, send them a letter stating that the old power of attorney is no longer valid, and enclose a copy of the new power of attorney.
If the old power of attorney gave the agent power to deal with your real estate, you may wish to record something with the registry of deeds indicating that the old power of attorney is no longer valid.
I don't generally recommend that you simply revoke an old power of attorney without signing a new one, but in some cases it may be necessary to do so before you are able to sign a new document. In that case, follow the steps above to put all parties on notice that the power has been revoked. Also, check the document to see if there are additional steps you need to take to notify the agents that they no longer have authority. As soon as you sign a new power of attorney, be sure to let the new agents and your banks, etc. know.
As always, your power of attorney should be drafted by an attorney and you should put care into choosing who to appoint. A power of attorney is a powerful document - which is what you want when you need someone you trust to be able to handle your property when you cannot.