I've always found the best way to get extras/perks/better service ect... is to swing by the galley and strike up some small talk for a few minutes, and pay a compliment or two. Not flirting, just being nice - key difference.

Working in service is hard work - especially hours on end in a metal tube with several hundred other people, and lets face it, there are many difficult passengers these people have to deal with every single day. A kind word goes miles and you'll be surprised what you can get in return. I'm sure chocolate bribing would be just as effective, but not as fun