I stretch their neck when I will be working on barrels. I get two treats and let them see the treat and then hold the treat by the stomach ( they will follow the treat most likely) then let them have the treat by the stomach where you had them stretch their neck. Do that to both sides
Before every ride I always start by picking up my horses leg and slowly pulling it forward and then stretching it back, on all four legs. For there neck I bend it around me and down between there legs, usually using a treat to encourage them to stretch. There are also many types of other stretches you can find on the internet or you can talk to your vet/ferrier fir more info. Hope this helps!
My horse have a sore back every now and then. My chiropractor said that we should stretch before getting on her every time. He instructed me to guide her head to the right and left side and between her legs with a carrot. Stretching between the legs was the essential one among the other because it foces her to stretch her back really good. She loves it and the first thing she does in the cross ties she put her face between her legs looking for a carrot. Finally, the chiropractor told us to do a lot of gymnastics work.