Better efficiency with rack and box?

Hello!

I recently bought a Yakima rack (50" corebars, 46" wind fairing, baseline towers) and cargo box (Skybox 12) for my 2007 Prius and was wondering if anyone had advice for better efficiency.

This week I went on my first trip with the new rig to Baxter State Park, 300 miles round trip with over half being 75 mph highway driving. On the way there I made 34 mpg in the rain, and 37 mpg on the drive back in dry conditions. I, of course, knew that there would be a loss in miles per gallon with the addition of these accessories and am completely comfortable with that; I love my average 48-52 mpg but I also love adventures with lots of gear and was curious as to what I might be able to do to get the most out of my gas.
I saw postings on here from people that were managing to have an average mpg in the 40s with similar racks and cargo boxes.