Use less accelerator. Stay out of the drag race from the lights and you will not only save up 30% on fuel costs, you'll also be a better road citizen and drive in a better frame of mind.
Pump up your tyres and check the suspension / wheel alignment. Maintaining the correct tyre pressure can reduce your fuel consumption by 10% or more and will increase the life span of your (expensive) tyres. Worn shocker absorbers or bad wheel alignment will also mean you will be replacing tyres much faster than you should.
Regular Servicing. Fix small problems before they become larger ones and you will save on expensive repairs. A well maintained and properly tuned car will also use less fuel.
Run your car air conditioner in winter. Might sound strange, but just a few minutes every now and then will circulate the system lubricant and prevent the many seals and hoses from drying out (which can lead to expensive gas leaks).
Regularly check your coolant ... not just the level, but also test the acidity and if there is any stray current which will cause a very destructive form of corrosion that attacks the radiator, water pump and other vital components.
Make sure your drive belt is regularly replaced or checked and tensioned correctly. A slipping belt will deteriorate quickly and, worse, a broken belt will cause catastrophic damage to the engine.
Use it less. It's stating the belling obvious, but a conscious effort to find alternative ways such as walking/riding, sharing a ride or using public transport, even occasionally can help you save money, save the environment and improve your health.
Regularly conduct a load test on the battery. This is a simple test that any workshop (and a lot of spare parts or tyre shops) can conduct in just a few moments. A failed battery is not only a massive inconvenience, an emergency replacement can cost a lot more.
Don’t buy a lemon. This could be the most expensive of all car costs. If you buying second hand, invest in a pre-purchase inspection.