Some of the devices I use when I have no helper is squeaky toys (either one designed for babies or dogs, either works). It is hard to work a camera and squeeze the toy at the same time. Since I wasn't blessed with three arms, I put it on the ground and use my foot.
Now my favorite and most-used is actually the cheapest and always with me. I carry it on my phone, so if you are like me, you are lost without it. I use an app called Horse Shaker Deluxe, which makes real whinny noises. Now I don't have to make my own noises and get those strange looks. Sometimes it works too well and the horse might get a little uncontrollable while trying to figure out where it is coming from. I love it when the horse's owner looks around to see where it is coming from; it's that real sounding.
When using these items, never show them to the horse until you are ready to take a shot. Horses get bored easily so if you show it to the horse before they are ready, they already lost interest.