Make sure your fridge freezer isn't next to any radiators - this can really affect how well they operate as the grill at the back a of fridge needs to be able to give off heat.
Keep the door shut as much as possible. Don't let the kids spend five minutes deciding what they want with the door wide open. Each time you open the door you have to recool the air in the fridge making it work harder.
Check the door seals are intact from time to time. Clean them if they're dirty and if they get riupped or torn then it's time to repair them or buy a new fridge.
Defrosting is a pain if you've not got an auto-defrost option and even then sometimes the ice builds up in the freezer making it a necessary chore to spend some time defrosting. Eat as much frozen food as possible before deciding to do this - run down frozen supplies. If you can't empty it entirely of food then why not get a chill bag or box and freeze some picnic icers and put food in there whilst you defrost.
If your fridge gets grubby give it a clean. Don't use washing up liquid as it's too highly scented normally for cleaning the fridge. wash with a solution of bicarb of soda and warm water to remove smells. Some people keep a tub of bicarb open in the fridge to take away any nasty niffs. I think it's better to remove the itme causing the smell. Some people use a lemon in the fridge to make it smell fresh.
If you have a fridge thermometer then check it out regularly to see what temperature your fridge and freezer are working at.