I recently chatted with a very friendly postal clerk at my favorite post office, and she gave me some great advice about addressing international mail.
I have long known that it's best to put the name of the country LAST, but haven't understood why. Sometimes I'll get an address on Postcrossing where there are numbers listed after the country. I usually figure that the person who lists their address most likely knows best the conventions of addressing mail in their own country, but that is not necessarily the case if sending from the USA.
I recently got a postcrossing card to Russia, and I've altered the info, but let's say the person listed their address as
15 Petroskaya circle
or something like that. My friendly postal clerk pointed out that you always want to list the country last, because American postal scanners will scan mail from the bottom up. If they see a country name first, it's headed for that country. If they see numbers first, they will read it as a zip code. I don't know if I'm explaining this as well as I could, but I think I understand some routing difficulties with international postcards I've sent in the past.
So - if you're in the USA, and you're sending to mail outside the USA - always put the destination country's name last, and on its own line!
And there's your friendly postal tip of the day. I love to learn new postal facts and tips!
UPDATE 8/4/2011: Thanks to blog reader David Harrison for some really excellent tips. Please read the comments for the full dialogue, but at the heart of the matter he recommended FRANK'S COMPULSIVE GUIDE TO POSTAL ADDRESSES, which is really spectacular. Do have a look.