Deciding on a purpose
Even description for description's sake should have a purpose. Is there an important overall impression you wish to convey? A central theme or general point? This is your thesis; organize your essay around it. For example, you might describe your car as your home away from home, full of snack foods, changes of clothing, old issues of the Chico News & Review , textbooks, and your favorite music. Or, you might describe your car as an immaculate, beautiful, pampered woman on whom you lavish attention and money. Just don't describe your car in cold, clinical detail, front to back (or bottom to top, or inside to outside) without having in mind the purpose, the overall impression you want to create. To achieve this impression, you should not necessarily include all details; use only those that suit your purpose.
Example: Imagine that you want to write a descriptive essay about your grandfather. You've chosen to write about your grandfather's physical appearance and the way that he interacts with people. However, rather than providing a general description of these aspects, you want to convey your admiration for his strength and kindness. This is your reason for writing the descriptive essay. To achieve this, you might focus one of your paragraphs on describing the roughness of his hands, roughness resulting from the labor of his work throughout his life, but you might also describe how he would hold your hands so gently with his rough hands when having a conversation with you or when taking a walk.