I have found that command strips don't like heat or humidity. I put a couple on windows around the house. The ones on south windows that got hot failed and all the ones on the outside of the windows failed. as stated above, the surface has to be pretty smooth and clean and the package will give you the weight info.
Also, it's not a bull clip it's a bulldog clip:
http://en.wikipedia.org/wiki/Bulldog_clip