the major things that most people can do are
vacuum the dust off of the of the back of the fridge, and use a degrease so the element can give off heat more efficiently (this will take a load off your compressor, and let you run a few degrees cooler)
check the seals. obviously if it's leaking out cool air the compressor will have to compensate for that.
keep the freezer full. if you have a lot of empty space in the freezer, fill up some empty 2-liter bottles with water and use them to get it fuller. when you have empty space, all of the cold (and heavier) air dumps out on the floor every time you open the freezer. the fridge/freezer then needs to cool all of that room temperature air down to the proper temp. an added benefit is that you will have less freezer-burned food, since you won't have the outer bit of food thawed and refrozen every time the door is opened.
usually when a compressor begins to go it fails to start. you can often buy a bit of time until you need to replace it by replacing the start-up capacitor with a more powerful one. this simple fix got me an additional 3 years of use out of my last freezer.