Saladin appears to have some good advice here. 
I've never had trouble with my chickens eating well. I switched to pellets as someone of
this board said the pellets hold the nutrients better. 
Makes sense. 
My chickens are healthy and long lived. They like to snack of course, but I'm careful
not to give them snacks until they eat their breakfast first. 
Sometimes I'll give them some special treat later in the day, such as apples or sunflower seeks. I like to be sure
they get a fruit or vegetable several times a week. 
During laying season I give them a big can of dog food, using a beef base, for the extra protein.
In addition I have a large bowl of oyster shell available which they nibble as I can see the amount gets
reduced over the weeks. This way I know they are getting their calcium. 
So far, so good.