Growing up in a small logging town in Oregon, one of my first pairs of shoes was full-grain leather boots. All my Dad's friends were loggers. They had breaking-in a tough pair of boots down to a science.
The first thing they taught me was to wear my boots around the house with damp socks. Not dripping wet, just a little damp, and to wear them every morning for at least a week or two. The moisture helps soften the leather from the inside out.
I've been doing it for almost 19 years and it works every time.
In the video you can see I just finished breaking in a pair of Hanwag Alaskan's. They fit great out of the box, but like any full-grain upper leather boot they needed some softening up. So after I wore them around the house in wet socks for a week I took them to the trail. Hiking in damp socks, I find it takes 25 or 30 miles to break a set of new boots in - about half the time it'd take with dry socks.
I tested this method last season with two sets of identical boots. Damp socks took about 20 miles to feel good. With dry socks it took closer to 50 until I felt comfortable. With a schedule like mine, a short cut like this is worth its weight in gold. When you log over 1,000 miles a year, you don't have time for blisters.