I've done it.
Get to the ticket counter well ahead of scheduled departure time, at least two hours ahead. The only cost is the airport taxes -- depending on destination, it could be from about $30 to $60. I believe they will take a credit card, but have cash ready just in case.
The ticket agent will give you a ticket which shows you paid taxes and that your international travel documents (ie. passport) are valid. This ticket will also enable you to get through security.
Then, check in with the gate agent to fill out the jumpseat form. Be at the gate no later than one hour before departure. From here, it's just like jumpseating on any flight -- make your way to the plane when told to.
Be advised that the A flight attendant on the international flights tends to be very busy during boarding. So try to be as unobtrusive as possible when introducing yourself to her/him on your way to the captain. Plus, if you don't piss them off, they'll probably make your ride a lot more pleasant.
Also, a friendly warning: the wine tends to flow pretty freely on the international flights (especially if you sit in Envoy). Just be aware that as a jumpseater you are considered part of the crew and technically shouldn't drink. I only mention it because I have a friend who got in a lot of trouble over that exact situation. Use your judgement.
Of course, I'm sure the crew will be happy to take any bribes offered -- but I've never met a US Airways captain who would deny you a ride if there is a seat available.
Upon arrival, when you go through customs, you are supposed to go through the passenger line (not the crew line).
If you have a choice, try to ride on the A330.
While the 767 is very comfortable in business class (they call it Envoy Class), it's a standard cattle car in coach. The A330, however, is a comfortable ride where ever you sit. Even in coach, every seat has it's own personal TV with dozens of movies, TV shows and music available.
Have fun -- Where are you trying to go?