I always whistle when feeding the lizards, at least when first training them. Some don't seem to care much and respond more to me kneeling down and putting my hand on the ground. Others will just come up to me looking for food. A couple of the spiny lizards (including the one I think is Bill from last fall) will run across the yard if I whistle. I think this iguana will eventually respond to whistling.
One thing I'm really curious about is whether they are making the humans=food association with me personally or with anyone. Once one of them gets really tame, I'll have someone else try to get close or call them over.
(hashtagging these with #LizardReport so you can mute them if you get tired of me posting lizard pictures and videos )