1) Simply put, measuring heart rate gives you a window into how much effort is going into your workout. It allows you to find the right amount of work in each session. Those who go too hard get dialed back, those who don't work hard enough know they have room to work harder or find more challenging exercises. Over working is a no no.
2) Find a heart rate monitor you trust. At RD, we use the MyZone system. It gives a color coded guide to each heart rate zone based on your max heart rate. This makes sticking to the appropriate zones much more convenient.
3) At Results Driven, we program big lower body exercises (squat, deadlift, sled push) with non competing lower body exercises (press, plank, row). Before your biggest lift, make sure your heart rate is in the 60%-69% range. Before your next upper body exercise, let your heart rate get into the low 70's. Stop in the 80% range.