For starters, make sure that it does not sit out longer than two hours and make sure to chill it appropriately as well. Whether that's the turkey or a casserole, get it into the refrigerator quickly to keep bad bacteria from multiplying.
Put your food in airtight containers. Because air fuels the bacteria-- so you want to keep that air out.
The temperature of your refrigerator should be set below 40 degrees to ensure that the food will stay safe. You may want to get a thermometer if your frig does not have one. And when it's time to reheat the leftovers - bring the temperature back up to the level needed when cooked the first time.
The Academy of Nutrition and Dietetics says to eat the leftovers within 3 - 4 days. They also advise, when in doubt, throw it out.