It is important when working with your horse that you have and maintain your horses attention. You cannot expect your horse to respond when you ask them to do something if they are not focused on you with their attention. Now, at the same time, we cannot ask our horse to forsake their natural ability to be aware of their surroundings, without this horses would not have survived the years. So with that being said, it is okay if something catches the horses attention momentarily, but they must not linger, their attention must come back to you. Of course that can be easier said than done sometimes.
So how do you keep your horses attention and bring it back when they linger? First, Be particular about this, but don't be picky. Don't get to upset if they linger a little, and don't act like a controlling human, instead, if the horses attention lingers for even a second, just bring it back to you. Use your leg to draw the nose the opposite direction the horses is focused on if you can, if not, use your rein, after you leg of course. When doing this don't be upset with your horse, remember it is their nature, but at the same time correct it with no bad feelings.
Okay, now the hard part, being consistent. You cannot expect your horse to offer you their undivided attention, if you are not consistent in asking for it. You have to be aware of where your horses attention is, this can be a difficult thing for the human to get. They want to get up on the horse and operate it like a machine, turn it on and let it do the work. Unfortunately for the human, it doesn't work that way, you must be mentally engaged with the horse at all times. If your not, how do you expect your horse to be mentally engaged with you?
Lastly, keep it interesting, don't just go ride around the arena day after day doing the same ole things time after time. Mix it up, get out and ride on the trail or on some open country. Find a friend with some cattle and learn to work cows. Give your horse a job, but give him one he can enjoy, not day after day of sitting in a cubical doing the same menial tasks. Make his ride something he will find interesting and look forward to doing rather then dreading and checking out.
Enjoy the Journey