When I was moving around I used a trusted friend's address in the country where I was registered, plus a mail scanning service for letters. The friend held anything important until I passed through. It only works if the friend is very organised and very patient, and I bought her dinner a lot.
moving every few weeks and post has become a real problem. bank cards, a replacement driving licence from home, one parcel from my mum that went back to vietnam after three weeks of travelling around. how do you receive post when you don't have a fixed address? mail forwarding services? a friend's address? poste restante? pick-up lockers?