What temperatures should I keep my refrigerator and freezer set at?
Daniel is spot-on with his answer. I'll elaborate on it a bit here.

As indicated by his bottled water in the freezer trick, a full freezer is a happy freezer. The same applies to the refrigerator too. While I wouldn't put random bottles of water throughout my refrigerator, it's important to know that the fuller your refrigerator is, the more it holds its temperature when opening/closing, and the less energy you'll use. However, you don't want to jam pack it so full that there isn't any airflow around your stuff, because this can hamper the cooling efficiency as well.

The ideal refrigerator temperature is 35°F (1.6°C). You're not hugging the danger zone like you would be at 40°F (4.4°C), and you're distancing yourself sufficiently from 32°F (0°C) that you don't freeze half the stuff in your refrigerator. That said, the temperature within your fridge can vary rather significantly with normal usage.

The coldest parts of your refrigerator are the back, and the bottom. The back because the cooling element is there, and the bottom because warm air rises. If items you don't want frosty are getting frosty, then move them away from the back of the fridge. I would avoid putting items in the door of your fridge that are particularly sensitive to spoilage. The items in the door of your fridge can easily get as high as 59°F (15°C), and do so often. Putting milk and eggs in your door will significantly decrease their shelf life. Butter is OK in the little covered section in the door, because the door actually helps keep it's temperature a little better. You also generally don't want rock solid frozen butter. 

So, put your meats on the bottom shelf in the back, your condiments (mayo, ketchup, mustard, etc.) in the door, and put everything else where it fits.