If you need to get mail (or a package) while abroad it can get quite complicated. Fortunately, there is a nice little trick for that, it's called "poste restante".
Poste restante (French term meaning literally "remainder post") allows one to send mail directly to a post office. The mail will stay there (usually up to one month) until someone picks it up with an ID.
Usually the address looks like that:
This wikipedia page lists address formats for a lot of countries.