You want to communicate to the horse that you like it, that you're glad it's there, that you like to be around it. You're not going to just grab the horse and beat it into submission. In terms of horse-speak, grooming can be a powerful influence you can use to gain control and trust. Wild horse survival requires strong herd instinct. Mutual grooming expresses camaraderie among horses and helps wild ones bond into a herd.
Horses love to be groomed.
Use this to your advantage to make friends with a horse when you first start working with it and to study how your horse communicates things to you. For example, if the horse is totally relaxed and looking around and sometimes looking back at you then you got some good quality time going on. Pay attention as you groom the horse to see where it's sensitive areas are and where it really enjoys a good scratching. Horses often signal their pleasure by screwing up their upper lip or by arching or stretching their neck when you hit an itchy spot. There are horses with very thin skin who dislike coarse brushes but if you groom them properly without sudden moves using soft brushes and a degree of pressure that agrees with them, there should be no problem.