If I would wear a wrap a long time, like for a hike, I would cross in front over the breasts while doing a back carry.
But mostly, I prefer the rucksack style for the back carry. You can do all the same carries, just bring the straps over your shoulders and straight down under your armpits to the back, instead of crossing in front. I like this look much better.
Another thing I have seen on some website or other is putting a twist in the fabric where it crosses. So you would bring the fabric to the front, cross and twist it around an extra time or two, then bring the ends to the back. From the pictures I saw, it looks like this keeps the fabric above most of your breast tissue. Haven't tried it yet myself.