I sit in the pen in a folding chair every afternoon when I come home from work, I always give them leftover bread or fruit, tried yogurt but they wouldnt touch it. Spending time with them is the best way for them to train you..or is the other way around?
The first treat I gave my chicks was boiled egg yolk. I mashed it up to an almost powder and it looked just like their chick food. Every afternoon, when I got home from work, they would get a treat. They learned when I came up with the plate, to get excited. That's when I started putting different foods on the plate and now they pretty much devour anything I bring to them. They're 22 weeks old now and we're still just one treat a day and they LOVE to see me coming with the plates.
I use pretty much any fresh fruit for treats, especially watermelon and cantaloupe. I also found (surprisingly) that they love squash. Chickens are about as nuts as I am. Go figure!