Make it very clear to them that you do not want your hair to be chopped off. Just a trim. Be very clear and stern, but not rude. It might even help if you tell them something like, "I want my hair long (or possibly longer), and many times ive gone to get it trimmed, but the haircutter just lobbed it off. I do not want this and ill take my business elseware next time if it happens again." Ive never had this happen to me, but I guess thats mostly because ive been getting my hair cut by the person who lives down the road from me for longer than ive had long hair. I guess you could say she is a friend of the family. She doesnt do exactly what I tell her to do with my hair, but she does know I want my hair longer. I always have to do little touch ups after she trims it, but thats no big deal.
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"Discovering the basic goodness of human life, the warrior learns to radiate that goodness into the world for the peace and sanity of others. "
--Chgyam Trungpa