Don't know about you, but I've got about 5 tubes at any given time of mascara. Some are high-end, some drugstore, but all share one thing: They dry up. It may be because I live in a very dry climate, but here's something I do to revive them to a glossy, creamy texture vs. the dry, clumpy texture they've become.
Just run hot water in your basin and plop the tube (make sure it's closed tightly). Let the hot water run over the tube while you do something else (line your eyes, apply concealer, etc.). Then, after a few moments (about 60-90 seconds is enough), your mascara should be nice revived!