The most successful program I had implementing Trailhead, I went into Trailhead Tracker before even introducing the program and found my power users. I then took some of the free Salesforce goodies I had collected during the year and made two surprise award packages wit ha letter acknowledging the extra work they put in without being asked and thanking them for taking time to invest in continuing their education. Word got out before I even finished my induction and Trailhead on-boarding messages because people wanted to know how they could earn free Salesforce stuff too. From there a badge-a-thon was an easy pitch because everyone loves free stuff.