I didn't see that anyone has mentioned how they approach their chicks in their brooder.
One of the biggest mistakes people make with baby chicks is placing the brooder on the floor, thus requiring that the chicks be administered to by reaching into the brooder from above.
Chickens all have an instinctive fear of being attacked by predators from above. By reaching for your chicks from above, it reinforces this fear, and instead of becoming more comfortable with their humans, they become more afraid.
This summer, I installed my brooder boxes on tables with access in the sides of the brooders. From the beginning, the babies could see all of me, not just scary hands diving at them from above. They were tame right from the start, and all of them have matured into pullets (and two cockerels) that are very agreeable to being picked up.
In addition to making sure you always approach your chicks from the side and not from above, if you make a distinctive call when you are bringing them treats, they will forever after come, like little well-trained soldiers, when you call them. This is most handy, believe me! I did it by bringing them earth worms while yelling "Babies!" Now when I yell "Babies!" they are all at my feet instantly! It's very convenient.