"You can bend. You can twist. You can lift. But you can't do all three at once." (Back Pain!) - Drew Magary
Drew writes a compelling story about back pain from a normal person's perspective. We happen to agree with most of it. We tend to use a little bit different language, but he's offering some good advice.
You can probably get away with any one of the three, but when you start combining these motions, you are putting your back in real jeopardy. And even if you do too much of one, you can still be at risk of some serious back pain. So, sit up while reading this. Don't stay in a slouched (bent forward) position for too long. And if you drop your pen, turn your hips and shoulders to face the pen squarely before bending down to get it.
And regarding lifting, please note that as far as your low back is concerned, when you bend over to pick up that pen, your back is still "lifting" half your body weight, plus the pen. It's not so different from picking up something "heavy". It's a matter of degree, not an exponential difference. So bend at your hips and knees, and try to keep your low back as straight as you can. It may even feel like you're sticking your bum out, but if you watch yourself in a mirror, our bet is you're not.
And it's important to observe good mechanics all the time, not just when lifting something "heavy", or working out. Even getting in and out of your car should command your attention to proper body mechanics and lifting yourself with good posture and mechanics. All those iffy movements can add up and take their toll so that, without much warning, one day you wake up and your back really does hurt and makes you miserable.
We're here when you need us, but that doesn't mean we want people getting hurt. We'd prefer you didn't need us, but we've come to realize that bad things happen to good people and we all experience pain and get hurt sometimes. It's our job to work with people when this happens to get them back to their lives as efficiently and safely as possible.
Please pay attention to your posture and mechanics and take care of your body!