Each new person I've worked with so far has needed some help with maintaining a flat back, and for many, it seems that the cue "hips back" or "stick out your butt" work really well. Check out the illustration below:
Where the spine attaches to the hips does not rotate like your knees or shoulders do. So, if you want to have your back maintain that neutral curvature, you're going to have to tilt the hips in the direction that you're moving. When you bend down to grab a bar for a deadlift or clean, you're going to have to adjust your hip tilt forward to maintain that lower back curvature. Cues to consider are trying to point your tail bone up or pushing your booty back. Ultimately you're using your spinal erectors to keep position.
Just another way in which working out can fuel a lot of raunchy jokes.
Anyway, being able to maintain this proper pelvic tilt is also important in maintaining back position in overhead moves where over arching in the lower back often occurs. In these cases, many people are allowing their hips to roll forward. Here it's important to think about maintaining normal distance between your ribcage and your hips.
Have fun and stick your butt out!