Start by determining a fair market value for your car: most people use Kelley Blue Book to do that. Then, make sure your vehicle is reasonably clean - you don't have to spend a lot of money for a professional detailing, but at least make sure to wash it well and do a thorough job of cleaning out the inside (no crumbs, fast food wrappers, or unpleasant odors!). Finally, set up your online listing for maximum visibility: the more information you provide, including photos, the more prospective buyers you'll get.