Avoid using heavy products on extensions. This can weigh them down.
If your extensions are made out of real hair, consider applying a bit of coconut oil to the tips. It will help nourish them and make then shinier.
Note the material your extensions are made out of (synthetic or real hair). This will determine what products and temperatures you should use. Some synthetic materials can't withstand high temperatures.
Get sewn-in extensions lifted every few months.
You get what you pay for when it comes to extensions. If you know you're going to be wearing them often, pay a little more for better quality, longer lasting ones.
If you can't find extensions that match your hair color, dye your hair, as dying extensions ruins them.