#1. Fermentation Temperature Control
Everything else is debatable in improving your beer on what level of importance it has. So in no particular order these things will improve your beer...
-Process improvements like full boils and fast chilling or brewing all grain with step or decoction mashing when called for.
-READ Yeast, Style books, How To Brew, Tasting Beer, New Lager Beer, tons of books on brewing - the info WILL help
-Compete - it can be a pain in the butt, but competing has taken my brewing to another level and it was already good.
-Judge - become a BJCP judge. Evaluating beer will give you a whole new appreciation for beer and the ability to find nuances in them that was previously absent to your taste buds. Armed with that information you will be able to formulate recipes with information about how the beer will actually taste when it's done.