Postcards: Write them while you're abroad, then bring them home with you, and mail them from there. It will be cheaper, they will probably get to their destinations sooner, and you won't have to waste precious time on your vacation looking for a post office. The only disadvantage is, your friends won't get to see those cute foreign postage stamps.

Delivery: I suspect one major cause of misdirected mail is handwritten addresses by people with lousy handwriting. I believe we have actually taken a step backwards in this regard. Back when people used typewriters, it was a simple matter to insert an envelope into the typewriter and type the address. Now that people use computers, they often hand-write the address because they haven't figured out how to feed the envelope into the printer. (On my own computer, if I follow the on-screen directions exactly, the envelope gets printed upside down.) Meanwhile, the Postal Service is increasingly relying on Optical Character Readers, mechanical sorters, etc.

The US Postal Service estimates that 25% of all mail has something wrong with the address. See USPS - Business Mail 101 - Addressing - Delivery Address.