Here’s a fantastic video by Emily Larlham in her Dogmantic training series. She offers a complete selection of free positive reinforcement clicker training videos that are IMHO, priceless. Although the videos are targeted for dogs, simply insert the word horse when she says dog, and you’ll experience the same valuable results.
This particular video teaches what I think is an invaluable tool that I’ve used in a similar capacity with my horses if I see their attention begin to wander. For groundwork, I’ll make a type of kissy sound and then click/treat for eye contact with me, and under saddle, I click/treat for a flick of the ear in my direction, either from a voice cue, or a rein aid.
I love her explanation as to why a verbal no-no, or eh-eh, is ineffective.