Assuming you have a smooth metal panel without any striations, ribs, coating, etc. we have had pretty good luck with adhered guards from these guys. http://www.snogem.com/sno-gem-snowguards-models.html
You have to make sure you follow the directions pretty closely or they wont stick very well. We started using them cause I didnt want to add screw holes through a perfectly good roof panel.
Also, Broken has some good ideas too. I have gotten into more than 1 argument with a contractor when I tell them I would be happy to install the snow guard system after they have it designed.