Be nice to us, don't call us woolly jumpers (at least not to our faces), explain your interest, and you may be invited to the flight deck. Please understand, though, that there are many reasons why it may not be possible. For example, if the trip is being used for training or examination, or there are staff occupying the jump seats, you may not get your wish. Don't take it personally, and ask again next time. Best of luck!