5. Keeping too much junk in your trunk
There are times when having a few bulky items in your trunk is a sensible thing, like carrying kitty litter if you're going out in wintry conditions, or plenty of extra blankets if you're an anachronistic criminal potentially disposing of... things. The rest of the time it's just dead weight, which A) kills your fuel economy and B) upsets the balance of the car, which could become an issue if you have to suddenly change lanes.
6. Tailgating the car in front of you like a big jerk
Everything else being equal, staying close to the car in front of you would result in better fuel efficiency, since you'd be driving in the lead car's wake (i.e., this is why you so often see race cars drafting one another). BUT, out in the real world, the vehicle in front of you tends to change its speed as the driver's attention fluctuates, and the inconsistency of having to speed up and slow down wreaks havoc on your MPG. Not to mention that tailgating is dangerous and a dick move.
What to do instead: Pick a reasonable speed and distance, and set your cruise control; over any length of time, your car will do a vastly better job of maintaining a constant speed than you can, and that's pretty crucial to fuel economy.