We are full time and have an annual membership at Americas Mailbox. They do have quarterly plans that would work for people that are not full time, so you would just do a change of address prior to hitting the road, and then again when returning home. We use their Gold plan where they are authorized to discard our junk mail, so we don't have to pay to ship us junk that we would just throw away.
http://www.americas-mailbox.com/home/plans/gold