2. Slowing down for corners
Fun fact: every time you use your brakes, your car's forward momentum is converted into heat, which then just radiates off into the ambient air.
What to do instead: Pretend you're a (speed limit-abiding) race car driver. Here's a primer on how to do so to save fuel. Be safe about it, but be smart, and you'll have a ton of fun in the process. I guarantee it.
3. Braking hard for a red light
Just as with approaching corners, braking heavily for a light that catches you off guard is bad for your fuel economy... not to mention your safety.
What to do instead: If you look far enough ahead and are properly maintaining your awareness of what's going on around you, you can often anticipate that a light is about to change, especially if it's one you see often enough to know how long it lasts. That gives you the chance to coast for longer.
4. Blasting your air conditioner with the windows down
To be clear, rolling your windows down on the freeway and blasting your AC on max in the middle of summer is nowhere near as bad for your mileage as it was in the 1950s, when that combination could result in a sudden feeling of weightlessness for your fuel needle. At most, you're looking at a very small percentage, but hey, in the quest for more money, every little bit counts.
What to do instead: Pick one or the other, but not both.