Hold them and give them treats. Daily. My ducks love frozen peas (defrost them first if you want...mine eat them frozen lol) and dried mealworms and old fashioned oats.
But also...we handled all our ducklings daily until they were fully feathered and moved outside, and now at almost 2 years old, none of them really enjoy being held (though my children hold them regularly anyway). I love to watch them and feed them from my hands, and they love to follow me around, but I don't force them to endure my petting if they don't want to be pet, ya know? I've got cats for that.