How do we carry our babies without ending up with an aching back?
We should use our muscles to keep the weight of our own upper body (and off our little ones') balanced. This doesn't mean having an absolutely straight lower back, like a military "attention" position. There should still be some natural curve to your lower back.
Do yourself a big favor. Hold your upper body weight up with your abdominal muscles; don't tighten your abs just use them to move them to a neutral spine. Your ribcage should be parallel with the floor.
When you are tired or have been standing for a while, it is easy to compensate by absorbing the load on your lower back. You stick out your belly, your ribcage points upwards and your hips tilt toward the floor (if you need a visual here's a picture of lordosis). If there is a bar stool or a rock around lift up one leg and take a load off and rest. If not, just tell yourself-
Wearing your baby high will also help keep the weight off your back. Facing your baby out will prove for an awkward load and almost force you to stick out your belly and your backside and let your lower back absorb all of the weight. Having the fabric extended to the back of baby's knees and making sure that baby's bottom is higher than your belly button, while your baby faces you, will bring the load closer to your core and better equip your body to carry your baby properly.
Happy back= Happy babywearing!