If your fridge/freezer looks like this you probably aren’t living as green as you should be. An empty freezer is much harder to keep cold than one that is packed full. Keep left over milk bottles and fill them with water, then use them to fill up the empty spots in your freezer. Same holds true for your refrigerator side as well.
I like to keep extra bags of ice from my ice maker in my empty spaces. That way, next time I have to fill up an ice chest, I don’t have to stop and buy ice, I can fill it up right there at home from my reserves. You should also be using the lowest temperature setting you feel comfortable with on both freezer and refrigerator. A good rule is to take it down 2 notches from where you are now, if you usually keep it at 8, try 6. It shouldn’t have a noticeable impact anywhere except on energy savings.
Also as you buy new appliances, opt for the ones that are most energy efficient. If your appliances are more than 10 years old, purchasing a new energy efficient model may be the way to go. Replacing a 20 year old fridge with a new energy efficient model will reduce your carbon emissions by one metric ton per year!