I ask silly questions.
"What's on my shoulder? WHAT IS THAT?"
"Who's messing with my hair? HEY, WHO'S MESSING WITH MY EARS!?"
They look. They giggle because there's nothing there and they think I'm crazy and silly.
I basically ask them a question often, even 3 year olds, where something is or what something is that is in the direction I want them to look. I find you or parents yelling "Hey!" "Smile!" "Over here!" Just is an exercise in futility. But when I just casually ask, "Woa, what's on my right shoulder!? What is that thing!?" Bam, eyeballs where I want them for just long enough to expose.
I always take props or a game too. Something. So if they get fussy or scared, they have something interesting to fiddle with. Often times it's fruit or vegetables. Corn, cucumbers, squash are amazing and cheap and a nice snack.