The way I usually do it is....
1 - if possible get a seat as close to the front as possible (a lot easier on BA eticket flights
)
2 - Try to be one of the first on the aircraft (ie as soon as its called get in there)
3 - Make eye contact and SMILE at the crew (regardless of sex) then ask them if you can bother them for a second.
4 - say you're a ppl (wannabe) whatever and ask if it would be at all possible for a jumpseat........
5 - pray. pray a lot
Hope this helps
B