I'm kind of wondering if it just may be possible that your techniques are getting better by exactly the same amount of effect that the temperature is having on the MPG - they're just canceling each other out.

Have you tried using low gear more under 40 miles per hour? I've found that has been making a big difference since I started using it a little while ago (which isn't saying much really, since I've only got 600 miles on my FEH and am on my second tank, so there's lots of room for other factors to be in the mix). It gets a bigger charge into the battery faster when you need it, and it's easier to get it into EV mode. And I've noticed in just the last couple of days that I'm able to accelerate a bit harder before the ICE kicks in in EV than I was at first, and I can hold it in EV at a little bit higher speed than I could last week, although I think I contribute that more to it breaking in than my techniques getting better - but it could be both !!