I think this is called "driving with load" or DWL, and I agree it's much easier to attain and maintain for long periods. Basically if I can't keep the instantaneous display at 100mpg, I want to keep it at least in upper half (over 50 zone).
On level roads I've found a small release of the gas pedal then a small tap, then frequently repeating that release-and-tap will keep the instantaneous needle floating somewhere in that upper zone. Downhill of course makes EV glide easy, so like PCK I always take advantage of it.
But then on some days

the instantaneous needle is just all over the place--it dances around on the right a little but just will NOT stay, or plunges back down to the left. I take my foot off the gas and all that happens is I slow down....grrrr.