Generally, chicken portions and breasts are less expensive if bought in larger packs, so straight after purchase split them into individual servings, wrap them well and freeze any that you won't be using within a day or two. They will keep for up to 3 months in the freezer.
Defrost overnight in the fridge and always check that chicken is thoroughly thawed before adding to the slow cooker.
You should also make sure that larger pieces such as quarters and thighs are thoroughly cooked before serving by piercing at the thickest point; the juices should run clear and not be at all pink, or test with a meat thermometer.