The most important thing is to pick a consistent sound to call them with. I use a kiss kiss sound, mixed with (apparently) baby talk. I was a little embarrassed when I realized I did this. My neighbor told me that one day she was talking to her baby when her son came in and said "Why are you talking to him in Jenny's chicken voice?"
So there's that. And then the second thing is to use the sound in conjunction with food. Chickens eat pretty much everything but their favorite thing seems to be my children's left over chocolate cereal. Yum.
I don't always give them food when I call them though. It's not really on purpose, sometimes I just want them to come when I don't have food. I also don't give them the same reward each time. Sometimes they only get a few veggie clippings and sometimes they get a mother-load of leftover rice. This kind of unpredictable rewarding makes the call even more effective. Think gambling!
Chickens are smart. My kids try the kiss-kiss call and it doesn't work. They only come when I call them, because they know I'm the one with the food . . . most of the time.
So get your baby voice ready and some yummy treats, and give it a go!