When I first started caching I realized I needed experience to find geocaches, especially well-hidden urban hides. The only way to get experience, I reasoned, was to cache often and try to find as many as possible. The goal was to find 100 caches a month for my first several months in the hobby. I now realize that wasn't sane. I had fun, learned a ton, but eventually grew tired of trying to reach a magic round number by a specific date.
The best milestone was a recent cache. I tried to find this urban cache 3 times when I first started caching and DNFed every time. Six months of caching experience later I tried again, walked up to GZ, and spotted it immediately. Now perhaps it was just a good day, or perhaps the cache wasn't placed back in the exactly right manner by the previous finder, but to me it meant my caching skill has improved and served as an indication of my progress in this insane hobby.