How many of you travel with extra "stuff" in your car all the time? Perhaps some luggage, folders, files, paperwork, clothing, or other items have been in your car for months. To make your dollar go as far as possible, make sure to remove all items that aren't necessary in your car- clean the trunk, keep only necessary items.
All that "dead" weight can reduce your fuel economy 2-5% for each hundred pounds. Maybe there's a stroller or car seat that hasn't been used in months. It'd be a better idea only to haul it when necessary as well because if something were to happen to you, any loose objects may become projectiles and injury you or passengers.
In addition to that, make sure that when you do haul extra items, they are safe. Hauling a trailer? Make sure to check your local laws- many states require safety chains that will prevent a major accident if the trailer detaches from the hitch.
Travel safe and smart, and make your vehicle as fuel efficient as possible!