Detangle your hair before washing it, and before going to bed. Use a wide-toothed comb, a wig brush, or a special looper brush. A looper brush is specially designed for extensions, and has looped bristles. Start combing your hair from the ends, and work your way to the roots in small sections. Never comb straight down from roots to ends.
If you just got extensions, wait a few days before combing your hair. If you must comb your hair, do so gently, and avoid getting too close to the joint.
Wash your hair every other day using a gentle shampoo. Lather your hair gently, and work the shampoo downwards, from scalp to ends. Try to avoid touching the sewn part. Massage your hair using circular motions.
Ask the stylist who did your extensions what sort of shampoo you should use. She might be able to recommend a shampoo that is best for your hair as well as your extensions.
Take care where you apply the conditioner. Start applying the conditioner from just below the sewn part. If you apply too much conditioner where the extensions connect with your real hair, your extensions may fall out.
Dry your hair all the way to the roots. When you get extensions sewn in, you end up with twice as much hair. It looks great, but it will take twice as long to dry. Make sure that you dry your hair completely, or you will risk the extensions loosening.
Consider spraying your hair with a heat protecting spray first. This will protect both your hair and your extensions from the hairdryer's heat.
Avoid using heat styling on your hair, such as straightening or curling. If you must heat style your hair, use a heat protecting spray and a medium to low temperature.
Don't tie your hair back just after you get the extensions. Wait a few days.
Cover your hair with a net or sleeping cap before going to bed. This will protect your extensions, and keep them from getting frizzy or tugged out while you sleep.