Call the United States Postal Service (USPS) directly, 1-800-Ask-USPS. Explain to the representative that you would like your local post office to hold your mail, so that an important letter is not delivered while you are away.
Send a request to have your mail held at the post office online, if you do not wish to call. See resources section for the "Hold Mail Service" page of the USPS website.
Find the location for your local post office if you want to fill out the "Hold Mail" form in person. The USPS offers a "Services Locator" option on the website, so that you can find your local post office.
Ask the representative at your local post office for a "Hold Mail" form. Fill it out completely and return it to the representative to stop the letter you are expecting once it arrives at the post office.