Could not agree more with the last post. Have a polite letter prepared, but never ask for the jumpseat - instead simply ask if it would be possible to visit the flightdeck at a convenient phase of the flight (they'll usually offer the jumpseat anyway), and ask the HEAD purser to pass the letter to the captain. (you should pass the letter on as soon as possible - at the gate is not a good time - they are busy, likewise whilst everyone is trying to find their seat. Just don't leave it too late, or the crew will be in the rhythm of pre-departure checks, and it won't be convenient)...
I would also suggest if a booking is made well in advance, that you write to the operator (eg British Airways or whoever), and ask their permission to visit the flightdeck. They won't give you permission (it's not up to them), but will reply in writing suggesting that you ask the Captain when on board the aircraft. If you paperclip this reply to your letter which you pass to the purser to take to the captain, it shows that
a) you've made an effort, and therefore are keen, rather than a half-hearted "I'd like to see the flightdeck, because I'm bored"
and
b) the airline have verified your address, so you are less likely to be some nutter!
I'd also suggest handing over your passport to the purser when you give him/her the letter. They probably won't take it to the capt, but at least it lets them know that you're serious.
Last year, I had the priviledge of sitting jumpseat on a BA 747 from Gatwick to Nairobi (the same route as the nutter incident) for the take off from Gatwick (at night) and landing at Nairobi (morning), where I saw wild giraffe by the airport perimiter!
They also passed a note to the next crew who were taking us and the aicraft onwards to Mauritius, who made a similar offer!