I have been sitting in the jumpseat for entire flights during PPL-training and now as a private pilot. I use one technique that usually works for me. Some times it donīt but if you want to enter the flight deck this is what you should do.
Never say why to the flight attendant! If you say you want to sit in the jumpseat, they will say no.
What I do is, I approach the cockpit during boarding and I say, "excuse me, may I just ask the captain a quick question?" Usually they say yes. If they wonder why, I tell them that I am a private pilot and I would like to know something about the aircraft bla bla bla. Once I enter the flightdeck I introduce myself, show them my PPL and explain my interest etc, and if I may join them. I have done this 6 times now and 3 times I have gotten the jumpseat. 2 said no due to SOP:S and one had a third pilot in the jumpseat.
Good luck!