Be careful of credit card interest rates which may negate your gasoline savings. Also verify that the stations you use don't charge higher prices to credit card customers
Stopping and starting the engine frequently will cause extra wear. Don't stop the engine if you are going to idle for less than a minute.
In very cold environments, it is recommended to allow the vehicle to idle and warm up, rather than just start it and take off. You might save gas, but your engine oil won't do its job until it's fully liquid, so you'll spend more money on overhauls.
Drafting is dangerous. All the fuel savings in the world will not matter a bit if you get wrecked while 'trying to save gas'. Safe driving habits will save a lot more money than risky driving, and maybe even save lives. Slow down. Be careful.
Nearly all gas-saving devices do not work, and some even decrease fuel mileage. Intake twisters, gas pills and fuel line magnets do not help mileage. Even if the mileage improvement claims were true, they often cost enough to negate any potential savings.
'Chipping' the car (changing/flashing the ROM in its 'brain') is usually meant to improve power, but often fuel savings are claimed as well. Be paranoid about it. It usually can achieve the power statistics it claims, but possibly at the cost of dreadfully expensive engine and drive-train wear and damage. The wrong chip (or a buggy version of a chip) can result in a dead car that's expensive enough to repair that it's 'totaled'. Needless to say, 'chipping' a car definitely voids the warranty.
Be very careful when shifting into neutral when going down hills. You may find yourself going a lot faster than you thought you would. This is actually illegal in some jurisdictions.
Be sure to keep to right lanes if you're not going to keep up with speeding traffic, and allow other traffic to pass.
If you drive slowly enough on a long enough trip to add hours, be sure to take extra breaks.
When you sign up for the 'loyalty' cards at your nearby store, realize that they do that to track your spending for their own benefit. That is not to say that you don't get a lot out of it. The savings can be substantial.