The people I used are http://vmfs.com/
-- Voyagers Mail Forwarding Service.
I used them while living in Saudi, while cruising, and after becoming a dirt dweller, until I had a permanent address about 18 months ago. Wehn I start traveling again, I'll use them again, unless they have gone into FIRE...
Sometimes I'd give them a standing order -- send everything except junk every 2 weeks, for example. Other times I'd send them 2-3 future stops and pickup dates, and they'd have the mail waiting for me each time.
In the 6-7 years I used them, I think there was only one delayed mail delivery, and that wasn't their fault. I think it was when the anthrax in the mail thing was going on, and one mail delivery was caught up in the delays.