Refactoring code -- changing its structure without changing its behavior -- is, perhaps, the most powerful tool for improving code.
Catalogs of Refactorings
Martin Fowler's [http://www.refactoring.com/catalog/index.html catalog], mostly from his book Refactoring: Improving the Design of Existing Code (0201485672)
Joshua Kerievsky's [http://www.industriallogic.com/xp/refactoring/catalog.html catalog], from his book Refactoring to Patterns (0321213351)