Saving money on gas can be done with two approaches*: reducing your consumption of gas, and reducing the price you pay per gallon or liter.
To reduce consumption, try the following: plan out your shopping so you can complete it all in one day, thereby reducing the need to pop out and get something. If you really need to get one certain ingredient, try walking or biking to the store. Carpooling is also a good approach. There are certain driving techniques that reduce gas consumption. The easiest is not going over 55 miles per hour (technically each car has its own optimal speed, but let’s generalize here). Most cars become less efficient at gas usage as the speed goes above 55mph. More driving techniques including being gentle (not driving fast then braking), not idling your engine for more than 30 seconds (if you’re gonna be stopped for longer, turn your engine off). Also make sure your tires are always inflated to the correct specifications else you will lose efficiency and gain nothing in return (unlike using an air conditioner, where you still lose efficiency but at least you’re more comfy).
The second attack vector is reducing the price you pay at the pump. Coupons are a great way – one gas station here is partnered with a grocery chain, when you buy stuff at the grocery chain your receipt includes a coupon proportionate to what you spend, usually a dollar or two. Dinky, yes, but that’s still a free liter of gas! As well some gas brands offer a loyalty card that either gives a discount on the price per liter or gives you reward points which can be exchanged for merchandise. Here’s the page of one of the biggest gas brands in Canada showing the stuff you can redeem your points for, which include gift cards and food at restaurants. So while it’s not explicitly reducing your gas cost, it is saving you money. And while the points sure are dinky and require a lot to have any actual value, if you’re going to be buying gas anyway you might as well get something for nothing.
*Technically three, you can buy a more efficient car, but the cost of buying a new car will outweigh the savings in gas for at least a few years, maybe more depending on your annual mileage, cost of car, etc.