there are a lot of options (we have a 25lber and use 3 of 4 different styles of sling each day) but the key is to have several good carriers and switch them out. any carrier that offers waist support (didymos and other wrap carriers, baby trekker, frame packs) will help your back. imo the baby bjorn is really hard on the back.
another thing is that with a growing baby it can be hard to keep your strwngth up unless you are carrying your baby a whole lot. sometimes i find that when tristan is in a growth spurt i have to carry him more often or while doing more strenuous activity to keep up with him. we eclusively sling him so we have to keep up! good luck!
Hi, I'm Tabitha. I'm a homeschooling mother of four: ds (11) dd (9) ds (7) ds (5) And I'm expecting a fifth in 2014! Find me at http://www.omelay.blogspot.com