I just recently found that all I need to do is call, "Come on you guys! Come on! Hurry up, come on!" They all turn on a dime and run straight for the pen and crowd around in a circle at my feet.
In the beginning, I'd shake the scratch grain jar and, because I'm impatient sometimes, I'd urge them to hurry. Now I don't even need the grain jar. It's very convenient if I need to gather them in in a hurry due to danger approaching from the forest and I don't have time to go get the jar with the grain.
When they were all tiny chicks, when I brought them a treat, I'd call, as I approached their brooder, "Babies! Baaaybeeees!" Even after they'd grown and gone to live in the coop, that call worked to get the youngest bunch to come to me.
Training chickens is easy and a very useful, too.