Keeping your mouth occupied helps - it's just enough to not be bored, but not enough to where you start losing focus. My personal fav is sunflower seeds, followed by tootsie pops or something similar. The salt from the seeds is pretty helpful in the heat too. But honestly, if you're glassing hard and thoroughly, it's a very engaging and methodical thing, and you shouldn't ever really get too bored. You've got a process, and you're executing on it.
In a way it's a bit like playing a game - look in the binos without moving them, use your eyeballs to hunt for shade that deer like to hide in, in a very methodical right-to-left pattern (opposite of reading, as it helps keep you from getting into rote habit with your eyeballs), pick apart shade, brush, rocks, etc, looking for anything from movement to ears, tines, hooves, etc, with extreme thoroughness. Then move the binos exactly one field-of-view over, and repeat. It's very much active hunting. Even just hunting for each pocket of shade in the glass, and feeling good with each one you spot, leaving no stone unturned. Your skills in spotting those pockets increase, and you get better.
It helps immensely to have had some success spotting deer, especially bucks, as it feeds on itself - the first time you spot a hidden buck that you absolutely knew was not there, an hour into a glassing session, the easier it is to stay in the glass and not convince yourself you're just chasing ghosts. So, power through until you've had a few experiences where you spot that ear-flick or tine, an hour into glassing a spot, and it gets easier. And, frankly, more engaging and exciting.
