Not all gas brands are equal:
It may all come from the ground, but some brands add additives to increase you gas mileage or prolong engine life. I know an owner of a gas station who does not even use his own gas for this reason. I have also come in contact with many street races who will never buy unbranded gas and are even partial to some brands. My thought is that if the Mobil or Shell station is within a few cents of unbranded gas, it is worth it. If you always use cheap crappy gas you will also need to spend 10$ on fuel injection cleaner every once in awhile.
What Octane?:
Try regular octane and if you can use the real crappy octane use that. Using better octane will mildly increase performance and gas mileage, but the increased price makes it pointless. Some insist on using better octane because they get better MPG and I believe them, but it still is not cheap. If it was the same price, I would say use it. I also hang out with people who need to either fill-up at the airport or use octane booster on their muscle cars. You don't drive those cars for their gas mileage though.
Gas disappears?:
People say to not full your gas tank and to especially not overfill your gas tank because it evaporates in the heat. I cannot see a lot gas just vanishing, but it is not hard to do these steps. I full my tank to save on extra trips to the station, but do not over fill it. I park my car in the shade and my black leather are not blistering hot anymore. Easy enough to do.
Shift Better:
Most cars are tuned to get the best gas mileage at posted highway speeds. If you are going 100mph, you may be covering more ground, but you are not saving gas. Use the correct gears. Do not use an extremely high gear thinking it is saving you gas because it is not. Use the correct gear and keeping the RPM's fairly low is best.