It does get easier! I actually cried one of the first times I tried to use my sling because it just was not going to happen, I was having all kinds of trouble with it and it was so uncomfy. Like almost anything, practice helps SO much. I don't have any problems with my back while slinging now, and only the occasional problem with my shoulder hurting when i just can't seem to get it to lay nicely. And ask you experiment and find new carries that work great for you, it will be even nicer! The biggest thing that helps me to get slinging down real well was to sling while I was sitting at the computer. No pressure about it "having" to work because you're at the mall and don't have any other way of carrying her. No one watching to make you feel self concious and embarassed when something looks wrong. And it was great for learning to nurse in the sling, ina private relaxed environment.
Congrats on gettng the baby to enjoy the sling, when I was pregnant with katrina just "holding" her was enough to make me too uncomfortable to do anything else! It sounds like you'll do fine at slinging once your baby is here.