Cook it right! Monitoring the temperature of meat and poultry as you cook is a sure way to avoid overcooking and make sure you've cooked the dish safely!
The Thermapen Mk4 Thermometer is the best! America's Test Kitchen says so, and so do I. More on temperature below.
Beef requires temperature monitoring for rare, medium rare, etc, but chicken is it's own animal (pardon the pun) because you want it to be moist yet certainly fully cooked. Sometimes a chicken breast can be so large you just don't know when you're at that sweet spot without a thermometer. My classic chicken recipe is a perfect example. You just aren't sure when you're perfectly cooked without a couple of pokes with your trusty Mk4!
Here's a great article from America's Test Kitchen that will give you even more info on cooking temperatures and the proper use of a thermometer.