Deadlifts! They are one of the best exercises for glutes, but it's so easy to do them incorrectly and injure your back. Most of you guys know this one as a Romanian deadlift - RDL. Here are some tips!
DO NOT ALLOW YOUR SPINE TO ROUND OR FLEX OVER
The main thing wrong with the before picture is the horribly rounded back. I remember doing RDLs like this in a gym before I knew what I was doing, and a trainer told me I would have back pain if I continued. If the weight is too heavy, it's OK to just deadlift with the bar only. In fact, when I went for physical therapy for a knee injury, the therapist had me RDL with a wooden stick to get the hang of the motion with proper form before eventually graduating to the bar. The back should remain flat and the neck should not hang down, which could severely injure you, esp if you have loaded a lot of weight.
DON'T WORRY ABOUT HOW FAR YOU LOWER THE BAR
The one thing I learned from working with a physical therapist is that how far down you can lower the barbell does not matter. If you have very tight hamstrings and can only lower the bar to just above your knees, that's OK. Do not attempt to further round your spine just because you think you need to lower the bar to your shins. As long as you are going far enough to stretch and load the hamstrings, you are getting the benefit of this exercise.
HOW TO DO THIS PROPERLY
Stand about hip- to shoulder-width apart with a slight bend in your knees. Grasp a barbell with an overhand, shoulder-width grip, your arms extended in front of your thighs. Lean forward from your hips, pushing your glutes back as you guide the bar down the front of your legs, stopping if you reach mid-shin or possibly higher. Keep a flat back and neutral spine with the bar close to your legs. Keep your weight in your heels as you return to start.