i have a non-specific greeting worked out for situations like that:

"excuse me, mr/ms blahblahblah? i don't mean to bother you, but i just wanted to thanks. your work/music has meant a lot to me over the years. *wide smile* have a good night/day!"

quick, complimentary, to the point, unembarassing. i have no desire for autographs or pictures, really, and i'd never think of approching someone who clearly was busy or irritated, but, yes. should the situation ever present itself, i feel that the above will come in handy. feel free to use it!