Before you take your horse out on the trail, it is important to warm him up. A few simple exercises in a round pen are sufficient, but also crucial in preparing your horse for the ride. 

The reason to do this is to maintain leadership over the horse and make sure he is always listening to you. Lead him to do several gaits and change directions, and watch to see if your horse is paying attention and obeying you. If your horse isn't distracted, he will keep his inside ear pointed toward you at all times.

Check out this video on warming up the trail horse.