The number one issue I concern when it comes to cooking salmon in a cast iron pan is sticking. There are two fail-safe ways to prevent this. The first is making sure your pan is hot enough before adding the salmon. The oil should be shimmering, but not smoking. If you want to test the heat you can (very carefully) flick a little water into the pan. If it immediately sizzles then you are ready to cook. The other is to let the salmon cook undisturbed for at least 5 minutes. This means no nudging or moving around, just letting the heat take care of business for you.