2-Leg Floor Bridge
Lay on the floor (on your back) with your knees bent and upper body relaxed. Raise your body off the ground and lift your hips until you have a straight line from your shoulder to your knees. Going higher than this puts unwanted stress on the low back.
Focus on tightening your glutes (primarily), hamstrings, and lower abdominals (as needed) while minimizing the tightness in your lower back. For added benefit you can squeeze your muscles at the top position and hold for a few seconds.
Once you start the set, your hips should stay off the ground the entire time. Stop if your low back gets tight or if the height of your hips decreases significantly. Perform up to 20 reps and breathe out when you are raising your hips up.