I've put a few thousand highway miles on my Volt recently, and I continue to better than average ICE/Hold milage if I keep some charge in reserve and reset the hold level whenever it drops below the red line.
Per Volt Stats, I'm at 69.19 MPGcs and that's all interstate/turnpike driving at speeds averaging over 70 MPH. The car is almost certainly reporting an inflated number, probably due to my technique confusing the software, but I do get significantly more range than reported by the range guess-o-meter ... an extra 50 miles in some cases.
As always, I will make sure to use all the electrons before stopping where I can charge.
As mentioned above, when switching to or from Hold mode, be aware of the inefficiencies of a cold ICE engine. Switch to Hold when the car will use the ICE long enough to reach an efficient operating temperature. The car won't cool off much if you use Normal for short bursts (e.g., decelerating/accelerating in and out of toll stations), but be mindful of turning off the ICE long enough to have it cool off (check the coolant information display to gauge this).
As always, Chevy did a good job designing the drivetrain. You'll do fine just driving the car. I'm only offering these suggestions if you enjoy squeezing out some extra mileage.