Upgrades

Does anyone have any tips for getting upgraded on a flight?
I know loads of people who've done it, and say you just ask at check in, but I feel silly asking in case they say no!
Worth a try though, I guess.
has anyone been successful?!

Tried...

..;but failed.
I had a friend who managed it once, she just smiled and asked very politely.
I think it's really just the luck of the draw.