I'm interested to hear the responses to this to see if there is anything new.
Basically, all I've been told is to build strength so you can spin in a higher gear.
I've found that if I really concentrate on truly spinning (instead of just hammering down on the pedals), I can increase my speed without my legs getting tired. Of course, it's taken me many months to get out of the habit of hammering. I've noticed over the last couple of weeks that I'm now doing this without thinking about it.