Dry chicken breast in slow cooker
A very common technique for maintaining moisture in meat is to dry brine it. That is, shake salt on both sides of the raw breast to the same degree you would to serve it. Wrap and let it rest for at least an hour. I like to add the salt the day before. Before cooking you can add other rubs or seasoning but be careful with store bought seasoning that is normally high in salt. Search on dry brining and you will find a lot of info.