Collecting is easy, of course, if you are passionate about one thing--say, wooden tramp art boxes or wind-up tin toys from the 1950s. But many collectors prefer a more eclectic approach--a less cluttered, livable mix of objects and styles.
Here are some broad parameters for organizing collections that don't rely on sheer numbers alone.
Strive to define your style in words. Is it romantic, contemporary, vintage, rustic, or Tuscan? By consciously determining your "style mission statement" you can refer back to it whenever you make a buying decision. If your goal is to express a "romantic vintage look using white" then you'll immediately know that buying a contemporary abstract painting won't contribute your goal.
Even disparate items look more cohesive when they adhere to a consistent color scheme. If you love the lines of a piece but the color's all wrong, consider painting or covering it, providing the piece isn't too fine. So, a black metal sconce can easily fit into your white aesthetic by giving it a few coats of white paint. Or, a clean white slipcover can cover outdated upholstery fabric.
Group items by similar subject matter to give them greater importance. If you like amateur artwork, for instance, focus on collecting landscape paintings or black-and-white portraits as your specialty.