I've always suspected that if you approach a celeb and talk about their REALLY old stuff or stuff that they'd figure NOBODY knew they were in, they'd appreciate it because it'd demonstrate that you're not just a fairweather fan. Plus, you know, talking to them like a human being and being polite with them rather than demanding something from them (as asking for an autograph might seem -- however innocuous it might seem from our side of the transaction).
That's the #1 thing to do, that is if you're a fan that does his / her homework. I'm one of those uber nerds that way though. I can usually name directors, music people, actors and what not at the drop of a hat. Not to mention I love 60's, 70's, 80's tv and movies.
I think I was about 15 at the time I met George Takei at a convention. I was polite but enthusiastic in wanting to know how it was working on Mission Impossible and with John Wayne in The Green Berets and other stuff. The look of shock on his face was priceless. :lol
My best and most touching memory was with James Doohan. I think he lived in Atlanta for a while and he was a pretty common fixture at conventions throughout the SE during that time. I've met him about three separate times, but the second time I actually caught him in the hall and had my picture taken with him (I had my TOS engineering uniform on at the time) and he was very gratuitous about it. A year or so later I brought the same photo to another convention with him and had him sign it, still in my TOS uniform. I kid you not, he had a tear in the corner of his eye. Of course, he's passed away now, and after that the photo went into a box of stuff I stored at my dad's warehouse on a move from college and the ass hats out there threw it out. Between moments of being pissed as hell, I actually cried.
Our local PBS station caught Sylvester McCoy on his way down to Miami for one of those celeb themed cruises. I used to do a lot of work there supporting Doctor Who on our local station. Anyway, the station provided a commemorative letter head that you could get his autograph on. Obviously, the whole gig was thrown together and the letter head thing was a bit cheap. McCoy was really nice, but he was kinda like "sure.. whatever". I didn't blame him :lol Anyway, I had a mint 1986 DWM from England with a publicity photo of him on the cover that I had him sign. That about made his day.
Anyway.. you have to give celebs a bit of credit, especially the ones that work the con circuit. It's probably a good part of their income. There's good days and bad days.. Take the most lame job you've ever had, the worst day you had there and then multiply it by 100. I'm sure that's close to how it feels.
People that make a habit out of being jerks are probably just jerks though and some stars let the fame go to their heads. However, all in all, there's a huge swath of people right in the middle where at the end of the day, it's just a job.