MMMhhhh... very hard my dear....
I've been flying several times with BA on long-haul but never managed to get more than a glass of champ and a newspaper... They have loads of BA crew travelling on standby and they are usually the ones getting the upgrade (logically). Unless you know one of the crew members personally I would say your chances are really really slim (say nil).
The few times I've heard of pilots being upgraded, they were travelling alone and they wore a business suit.
I also position very often with BA and even wearing my airline's uniform I never managed to get upgraded.
With AirFrance I even ended up sitting in a crew seat for the whole flight due to a mistake in the booking system, although there were loads of empty seats in Espace France.
My best advice would be for you to register with BA online as a frequent flyer and start collecting miles. You can then redeem your miles on a successive flight (for an upgrade from cattle class you need 25'000 miles on a long-haul). Unfortunately your flight to Miami will only pay about 6000 miles so you'll need to travel some more. It's a long process but if you keep doing it for a while you'll eventually fly for free! Better than a kick in me front-bottom! (awww... I like that Ali G joke!).
