Hi,
I've been lucky enough to get numerous Jumpseats on BA 737-400's, a BM 737-300 and the best one of all on a BA 777 from LHR to Philadelphia and on the way back.
I've always just politely asked a member of the cabin crew on boarding, sometimes the captain says no if its a training flight or the captain doesnt like jumpseaters, but I would say that about 80% of the time I've been allowed in from pushback all the way to the end.
Just be friendly and polite, it works wonders
LP