This is one of my longer tips, and it more of a ten minute tip. It focuses on a very simple concept, executed in an elegant manner.
Sometimes when modeling especially in Subdivision modeling you end up with a loop of edges that is too dens for your needs. This can happen when multiple objects merge, or when you are reducing the diameter of a cylindrical object. The below video shows how you can take a loop with an even number of edges, and easily reduce the number of edges therein.