If you move to a new address you can assure that your mail is sent to you at your new home by filling out a simple change of address form (Form 3575) from the postal service. To get this form, go to your local post office and pick up a "Mover's Guide®" packet, available for free in Spanish and English. Form 3575 asks your postal carrier to forward letters to your new address. The post office will also forward newspapers and magazines, but you may need to pay extra postage.
If you have a case with a government agency like the Immigration Service (INS), IRS, or Social Security, you should contact them to let them know your new address. Most agencies have a specific form or card that you need to fill out and send in. Call the agency's toll-free number for more information. If you do not forward your mail, someone else will receive your important letters and could read them and/or destroy them. You may also miss important deadlines!
If you are moving away from your current address for a short time and plan to return fairly soon, you may want to keep you're a permanent address where it is, if someone you trust is there to pick up your mail and tell you if important letters arrive.
The post office also allows you to change your address online: https://moversguide.usps.com/