File a
temporary change of address form with the US Post Office and tell them to send your mail to a relative's house if they can put it in another envelope and send it to you.
Or send it to a mail forwarding service like Seilerbird suggests. Check with your local UPS store, some of them will rent you a mailbox (like a PO box) and for a fee will gather up your mail and send it to where you'll be.
It doesn't apply to you, but the Temporary Change of Address is only good for up to 6 months, or 1 year if you renew it. If you are going to be away longer than one year, you'll have to submit a Permanent Change of Address and you won't be able to change your address away from the forwarding company when you return home. This is because the post office won't sort through all of the mail going to the forwarder to find just your mail to send back to you.
As far as where to send it, most post offices have General Delivery. You (or your forwarder) addresses the mail to Your Name, General Delivery, City, State, Zip, then you go to the Post Office counter, show them your ID and get your mail.
Choose a place where you expect to be in a few days and have your forwarder send your accumulated mail there. It's best to choose a small town with only one PO branch, larger cities with multiple POs only have General Delivery available at one location, and it may be a ways from where you'll be at that time.
The Post Office also has their own Premium Mail Forwarding, but it's fairly expensive ($20 enrollment and $20 a week) and you have to tell them every Wednesday where to send your mail. It's on the same page as the above link.