Cast iron is naturally non-stick. However, for it to work best, it really needs to be hot. Make sure you let the skillet sit on the burner for a little while and let the heat build up in cast iron. No need to turn the heat on high, leave it on medium and give it time. Add a little oil before searing. Boneless, skinless chicken breasts have very little fat so they can be more prone to sticking. Don’t play with your food was the best advice your mother ever gave. Let the chicken sear completely before trying to turn it. The more you fiddle with these breast pieces, the more likely you are to have them stick. Once the chicken starts to brown and create a nice crust, it will come away from the pan without sticking.