Internationally
My experiences jumpseating internationally.
First, you asked about wearing a uniform and having a higher chance or not for an upgrade.  If you walk in and tell the lead flight attendant "hi, I'm Joe with ABC Airlines, mind if I check in with the cockpit crew for that?"  That should be enough of a clue for them that you are a jumpseater and wearing a uniform would serve no purpose.  I wouldn't bother; people are just going to look at you for 1-16 hours and you could be wearing something slightly more comfortable anyway. 
Continental is reliable for getting on the flight if there are seats.  No jumpseat listing issues, just do it at the ticket counter 1.5 hours or more before the flight.  They won't give a jumpseater business class at the gate, but the crew might upgrade you if there is a seat available.
United is fantastic now that they take unlimited cabin jumpseaters internationally.  The chances of being given a business class seat are high, even by the gate agent.  If they don't give you one, the United crews really try and take care of you (so I return the favor!).  They also seem to be very forgetful in charging departure taxes.  The only thing you have to remember with United is that you want to be listed in advance via the 1-800-UAL-LIST phone line.  Use the phrase "OMC" for jumpseating.  Hubs are able to list you but I wouldn't count on it, i.e. it may take a ticket agent at NRT a few minutes to find someone that knows how to enter it into the computer there.
Delta is good too, since listing at the ticket counter always has seemed to be reliable.  JFK crews seem to have a friendlier mentality as far as upgrading a jumpseater to an empty business class seat as compared to ATL crews in my experience, but I've non-revved on DL more than jumpseated so that is a very unscientific poll.  No offense to the south DL side whatsoever, but the former Northwest side of things at Delta is more jumpseater friendly in my opinion.  Another thing: do not use the 767-300ER for jumpseating on a flight that is longer than 9 hours unless there are at least 50 seats open (I'd say 25 but you're going to be after all other non-revs including buddy passes so a buffer would be wise), plus between Beijing and Tokyo even though it is only 2.5 hours (weight restricted!).
US Airways has been nice to me as far as offering me a good seat.  One thing to look out for though is there may be some confusion as to where they want you to have paid the departure tax to get back into the states.  They told me in Europe once that I had to have a receipt that I paid the tax in PHL, before leaving Europe on a flight to PHL.  Wrong.  I've since never used US Air for international travel, even as a backup.  That situation may have been an anomaly unique to that station, however (Rome).
If it at all makes sense with your travel plans, take Polar/Atlas.  Automatic comfortable seat, relaxed atmosphere, and if by chance you are bumped out of the jumpseat because they have to deadhead some of their own pilots on that flight, they will be very helpful on the phone in getting you on the very next available flight.  They will literally read you the list of options you have to get home, in multiple directions around the planet depending on where you are.  The guy was ready to list me and a friend from Hong Kong - Sharjah - Frankfurt - CVG if we wanted!  We stuck with HKG-ICN-LAX the next day though.  They met us in the pax check-in area and drove us to the airplane on the cargo ramp, and we cleared customs with the crew on the other end of the journey the same way.  If the flight is a two-pilot segment, the rest bunk will be empty -- they were nice enough to offer us those on the shorter segment and that was really great to lie down and take a nap.  
If I had to sum it up in one sentence, seats available: Only use USAir for leaving the US not coming back in , Continental and Delta will get you there, I wish Northwest was still around, take United if you can't take Polar or Atlas.