Consumer Reports just reported on gutter guards. I don't remember the name of the company that made the highest rated one, but it was not cheap. It had really fine mesh fabric.
Personally, my favorite is Gutter Brush. I had them in my gutters for several years with no problems. They look like oversized bottle brushes and just fit into the gutters to keep leaves, twigs, birds, baseballs, etc. out of the gutters. Anything that gets stuck in the bristles (like maple seeds) eventually rots away or dries up and blows away.