This is very personal and anecdotal............but I feel like I improved a lot after I had begun to do really hard intervals in the 3 minute or so range. Enough duration that you're not racking around sprinting, but enough intensity and duration that your body is going to really really really want to find the most efficient pedaling to cope with the work load. That also goes to say that during those efforts your body is going to want a nicely setup bike too in terms of fit. That's how I figured out some cleat position tweaks.

Just me. May seem odd, but seems the that kind of stress forced me to get it "correct" for my body.