I live in Colorado and (as you would guess) do a fair bit of mountain driving.

Don't use cruise control when going up a long, steep grade. The assist will usually not come on, and the RPMs will just get higher and higher to maintain speed.

Take it out of cruise, then to force the assist to come on, pulse the gas pedal and watch the assist bars rise. The RPMs will not rise as dramatically as when using the cruise.

At least this is my experience, and I regularly get about 46 mpg.