The last time I got my hair cut, I got into a conversation with the stylist about her little brother who engages in such severe tantrum behavior around getting his hair cut that his parents had resorted to trimming his hair while he was asleep. 
 For many families with kiddos on the autism spectrum, getting their child's hair cut can be every bit as traumatic as a trip to the doctor or the dentist.  Because it's safer to put off haircuts than medical appointments,  a child might not get a lot of practice at getting haircuts successfully and establishing rapport with a compassionate stylist as they do with their doctor.
Here are a few strategies that can ease the stress of haircuts for everyone.
. What's not to like?: Different kids have different fears/anxieties about haircuts.  Try to identify specifically what is most distressing to your child so you can decide whether to target that activity for desentisation or to eliminate it from the process.  For example, if the sound and feeling of the electric clippers is particularly stressful for your child with sensory sensitivity, you may opt to instruct the stylist not to use them while you continue to work on desentitization in the other settings and just have the stylist use scissors for now.   
2. Practice sitting in the chair: Arrange with a hair salon/barbershop to let you and your child come in and practice sitting in the chair for short periods.  While the child is sitting in the chair, allow them access to a preferred activity (toy, ipad, music). When you have the child get out of the chair, remove the preferred activity.  While the child is acclimating to the chair, attempt to add some snipping/spraying noises around the child and have the stylist (or you!) touch and lift the child's hair just like you would during a haircut.  After exposing the child to the noises, have him leave the chair. 
Cut a little: When your child has had some successful trials at sitting in the chair with snipping/spraying, then have the stylist attempt to cut a section of the hair.  Give the child access to the preferred activity while the stylist is working and then (before any protesting has occurred!), pause the haircut, get the preferred activity back, and give the child a break.  This will insure the child doesn't satiate on the item and will want to come back for more.  After a short break (a few minutes) have the child sit back down and continue with the preferred activity and the haircut.  Repeat 2-3 times until haircut is done.
4. Quit while you're ahead: If you start to see the signs that your child is getting ancy, encourage the stylist to quickly get to an acceptable stopping place and STOP. Attempt to end the haircut before any significant protesting has occurred so that you end on a good note and your child's last experience with that haircut is positive.
5. End with a treat: When the haircut is done (or you have decided it's done), give your child a special treat while he is still sitting in the chair.  Getting a particularly preferred and rare treat while at the hair salon/barbershop will further cement positive associations with haircuts for your child. 

Hopefully, these strategies will cut some of the stress out of haircuts for you, your child, and your stylist.  All children are different, but with some patience and practice, most kids can have a positive experience getting a haircut.  Please feel free to try these strategies or share some of your own!