If you have straight hair and want to wear a synthetic extension, add shine serum to the ends of your hair before applying the faux strands. I am often asked: How To Make Synthetic Hair Extensions Blend into Your Real Hair. This sheen the serum will give will help your real hair match that of the hair extensions since they tend to have a silky-plastic look to them.
The serum I often used when styling for shoots was Oi Oil by Davines.
If you have human hair extensions, you can do whatever you do to your own hair—and the faux strands, such as flat iron or curl them and add shine spray. Just be cautious to protect the hair from the hair extensions as you would your own to make them last longer. They need the same TLC and make sure to get them cut to bend your hair and trimmed if you are one to wear them on a regular basis as the split ends if they let run wild will tangle in with your own hair and WILL cause breakage and ratty looking hair.
The heat protection I like to use, and not just on my client’s hair, but on my very own hair is AG’s Firewall.
For more tips on long hair please read http://kerridawn.wordpress.com/?s=long+hair.