What conditions make butter last longer?

Proper refrigeration--at 40° F or below--is vital to ensure the best possible shelf life for butter.
Experts offer these tips:

• At the supermarket, leave butter in the chiller until you’re ready to leave.
• Before purchasing, check that the container feels cool and isn’t damaged.
• Get butter home and into the refrigerator as quickly as possible.
• Never expose butter to room temperature for more than 24 hours.
• Freeze butter in foil or plastic.
• Once opened, butter should be refrigerated in a covered dish.
• Butter absorbs other food odors, so keep it in the fridge’s butter compartment.

 
Source(s):
Assistant Professor Franco Milani, Department of Food Science, University of Wisconsin
National Dairy Council Factsheet: “Butter”