To get the most useful scent life from a car freshie, use it as directed and avoid unnecessary heat exposure. You cannot stop fragrance from being released while still expecting it to scent the car, but you can avoid treating stronger output as the only sign that it is working.
When practical, park in a garage or shaded spot. This can reduce heat exposure, although it does not guarantee a particular freshie lifespan. Follow the freshie's placement instructions and keep it off interior surfaces.
Do not put the freshie beside a heater or deliberately heat it to bring the scent back. A temporary increase in smell is not the same as restoring the fragrance it has already released.
Keep spare freshies in the packaging and storage conditions recommended by their maker until you are ready to use them. Opening several at once exposes fragrance you could have saved for a later replacement.
If you like switching scents before one is finished, check whether the seller provides storage instructions for a partially used product. Do not assume any bag or container is suitable for fragrance materials.
Evaluate the scent when you return to the vehicle after time away. Cold conditions and becoming accustomed to the smell can both make a working freshie seem faint. If it remains weak under normal conditions after repeated checks, it may be spent.
Avoid adding random oils or perfume to extend its life. Unless the product is designed to be refilled, extra liquid can change the material and create a mess on nearby surfaces.