Since my folks' house is surrounded by fruit trees they've always had problems with leaves clogging up sections of the gutters. My mom went to Orchard Supply Hardware (OSH) and bought 36" PVC snap-in gutter guards, which install as Stone described in his post. We only put them in the sections surrounding the areas where the trees overhang the roof. They've been up there for a few years and do an excellent job.

As I recall, they were less than $2 per 3-foot section.