Clean out the car completely. You don't want to have anything in the car that isn't something you are moving to college with.
Pack your things in uniformly sized square boxes if possible. Tape them all shut and label them with a marker. If you can get boxes that are all the same size, it makes packing the car easier because you'll be able to stack them like building blocks. Otherwise, you'll be packing the car like you're playing a game of Tetris. Pack as many clothes as you can in large black trash bags. This will allow your clothes to fit in areas too tight to fit a suitcase.
Fold down, collapse or remove any seats in the car that you can. Start with the maximum amount of open space possible. Don't forget the front passenger seat, either. Unless someone is driving to college with you, the only space you'll need free is the driver's seat.
Start with the large items. Because most dorm rooms supply furniture, your large items will likely include your TV, dorm refrigerator and microwave. If you're bringing a favorite chair or other piece of furniture, put that in before the small appliances.
Pack the boxes next. Do your best to pack them so that there's no air or visible space between the boxes. Don't forget to utilize the floor space between the seats, and the spare tire well in the trunk if it isn't being used to house a spare tire.
Pack your clothes next. Slide the bags in between the boxes and the roof of the car, or in other areas where you have visible space. Squeeze the bags in where you can; you can always iron the clothes if they come out wrinkled.
Pack the other items last. There are always things that don't fit in a box well. This may be a toaster, a wall decoration you really like or your Mickey Mouse telephone. Pack these items last so you can manipulate them into the space you have left.