Horses show their interest and their attention with ears. When you are photographing horses, you want to show the horses at their best: interested, attentive, friendly – all those things, so you want to photograph them with their ears up.
There are many different ways to accomplish this task. First, consider the horse that you are photographing. Is the horse nervous or uncertain? Then you might not want to scare or overwhelm him with stimulus that might be too much for him, like a waving flag. A better “ear getter” for this horse might be a can with grain in it that you shake gently. This works especially well for the food-motivated horse (which is most horses!)
I also use a flag – a metal stick with fabric on the end – you can also tie a plastic bag to the end of a whip or stick. Make sure you don’t startle the horse too much with this – start slow. Wave it gently, and then if the horse responds well you can get a little more vigorous. Safety first though – you do not want to scare your subject and put the handler into an unsafe situation. Some horses are unimpressed by either of those and so you have to be flexible and adapt to your subject. Stallions will often look in a mirror and prick their ears and puff up impressively. Another thing you can try is have another horse walk behind you – the horse will be interested in looking at the other horse.
When your are photographing, it really helps to have another person be in charge of “getting ears” for you so you can concentrate on taking the photograph. However, I often end up having to juggle my bag of tricks myself. When you are using a flag or shaking a grain can, don’t do it continuously – the horse will get bored quickly and tune it out. Use it for a short time, then stop, then try again.