It is really two problems: maintaining residency in your current
state (assuming you want to do that) and dealing with collecting and forwarding mail.
For the first we use a relatives address in the state. While this should not really be required, we've found it really difficult dealing with various state agencies (DMV, Voters Registration
, etc.) if we don't have an address in the state. With each of these agencies we also put in a mailing address that is our mail forwarding service. They happen to be out of state.
For mail handling and forwarding we use St Brendan's Isle in FLA. They are reasonably priced and will scan any letters you get to allow you to see them online. Great service. FLA is not our state of residence.