Like with virtually everything, cars have shelf lives. Some cars have a life in years (usually between 10-20) while others start to stop working once they have completed a certain mileage. Do not always wait for your mechanic to tell you that there are serious problems with your car, if you hear an annoying noise, the chances are that you will start having problems soon.
If your car is rusting, you should consider getting rid of it as soon as you can. Rust is hard to get off a car and also spreads rapidly. Once your car starts rusting, its value and its safety starts to decline rapidly.
If your car is regularly requiring repairs, you should also consider parting ways with it. Most problems with a car can be repaired but you should always try and make sure that the repair is cost-effective. If the car requires a serious repair to the frame or an engine, you should consider getting a new one.
Safety is one of the most important things when it comes to cars. If there is a problem with your car, check with your mechanic to find ways to make it safer again. Even if you are planning to sell it, increasing its safety will also increase its price.
Work out how much money you are spending on petrol. If you feel that you are spending too much, you should consider switching to a more fuel efficient vehicle.
Most people buy cars when they are aware that it is likely to cost them a lot of money to repair. If the money you are spending on repairs makes things financially difficult for you however, the you should seriously consider parting ways with it and getting a new one or using public transport instead.