[header] How to brush your dog without tickling it [title] Accept your dog's natural scratch reflex. [step] Any time you touch a dog's sensitive areas, it creates a ticklish/itching sensation for the dog. This causes a reflexive response in which a dog will kick one or both of its hind legs. 
Your dog cannot control this reaction, much like how you cannot control the way your leg kicks when a doctor strikes your knee. [substeps] You cannot train or punish this reaction out of your dog; you simply have to accept it and work around it when brushing.