4. OPEN THE WINDOW AND USE THE VENTS, NOT THE AIR CONDITIONING
Even with how much common sense this seems to be, I've had people tell me, "I never even thought of that..." Really, it's not that hot out there, is it? The reason most people won't use the window is because they "can't hear their radio". Boo-hoo. It's either turn the radio up and deal with a bit of street noise, or spend more money on gasoline.
Speaking as someone who lives in an area where it IS, in fact, THAT HOT: It gets up to and over 120 degrees (F) around here in the summer. The AC sometimes just takes the edge off of feeling like you're cooking. The rest of the year, though, I agree- 90 degrees or lower, just to ease the tax on your tank, try lowering the window and sticking it out. :D
Tips for saving money/gas:
Like what a lot of people here have said, WALK. This gets a little tougher in the summer for people who are in hotter areas, or winter for those in colder areas, but generally speaking, humans can do a lot of walking! It takes a little longer to get around to where you're going, but it saves on gas and will help you feel better. So! It means spacing out your time a little differently- if you know it'll take you half an hour to walk somewhere, leave half an hour earlier than you would've in a car. If it's a quick run to the convenience store just a mile away(Yeah, I know that's a daunting walk for many people), try hoofing it. It gets easier the more you do it, and you'll save money.
For those who are not able to be on their feet for that long, bikes are also useful.
I wanna point out, though, that I'm aware not everyone has the luxury of being withing walking distance of everything, or even walking distance of a bus. That of course is subjective- I live a mile from the bus but still take it when I can, including walking TO it- but the advice I'm offering applies to those who ARE within the walking distance of these things.
If you're in the age bracket/money bracket where this applies, getting a hybrid vehicle helps A LOT at the pump. My father owns a hybrid prius, and he regularly gets a minimum of 45 MPG in that thing. He can often get 50-55 MPG, if he drives smoothly(slow acceleration, so on). It costs a lot in the short term, but if you're planning for the long-term it WILL make a difference.
And just in general- if you're comfortable at 65 degrees, try pushing the envelope on your comfort by kicking it up to 70 degrees on your AC. Or even 75. Or higher. If you can tolerate it, it will save you a BUNCH on bills. Use fans, or ice from the fridge- I make makeshift ice-packs with a towel and some ice, myself- or take a cold shower, or dunk your head in cold water. Try to get used to the heat rather than the cold- remember, you can always put something on if you're cold, but there's only so much you can take off if you're hot!
Also! Close blinds/curtains on windows, that will cut down on energy usage for the AC. The darker you can afford to keep your house, the cooler it will be.
Turn it off when you leave. The computer, the fan, the light, the TV. Every small thing you turn off saves you on energy bills. Use sunlight for lighting if you need it, instead of the desk lamp. Maybe even, instead of watching TV, read a book. Write. Draw. Have at non-electrical activities. It's surprising how much we've forgotten we can do to entertain ourselves/each other without being wired into some electrical device.
Buy a water purifier for your faucet, to cut out all the yucky stuff that comes from the tap. Then, buy one (Or more!) bottles of water, drink them whenever, and just refill them from the tap. That saves on spending a lot on bottled water, and it recycles instead of just dumping.
Short showers, or take baths. Conserve your water, AND how much water your waterheater has to, uh, heat. :3
I know that some of what I say isn't applicable to everyone, and I am sort of talking about "here in the US", because that's where I am. I can't speak for other countries because, well, I don't live there.