Even the most state-of-the-art baby carrier that has incredible weight distribution may not be enough to keep your back from aching. The trick is to remember that you need to be proactively focusing on keeping the weight you are carrying off of your lower back. BUT...
How DO you carry babies without ending up with an aching back?
By using certain muscles to keep the weight of your own upper body (and of your 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 to oblivion, just use them to move your back 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 there is a bar stool or a rock around place one leg on top to take a load off or just sit and rest.
If not, just tell yourself- "I will not allow my lower back to slump under my upper body and the weight of this sweet babe that I am carrying!"
Wearing your baby high and turned inward will also help keep the weight off your back, whereas 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.
Last two major tips when carrying in a wrap:
The first step to avoiding back pain before even putting your baby into the wrap is tying it nice and snug (the wrap shouldn't have any slack in it and feel like a tight t-shirt).
After placing your baby into the wrap, make sure the fabric is extended to the back of your baby's knees and that baby's bottom is higher than your belly button but lower than baby's knees.
These steps are key and 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!