Keep it simple, and focus on simple carbs. You need a little fat & protein, but mostly carbs. No fancy preparation - if it takes more than 15 seconds to start munching, leave it at home. Bring stuff you REALLY like. One of my friends brings pound cake on high altitude climbs. Is that a healthy, nutritious food? Hell no, but he knows he can get it down when he's up high.
Dried pineapple(my favorite), mangos, dates - items like that are quickly digested. So are fig newtons. Cashews provide a bit of fat. A PB&J on Kirkland's Multigrain bread (my favorite commercial bread) for the summit. A few hard candies to maintain blood sugar are tucked into one waistband (love those packs with a web pocket on each side of the waistband). A good place to get these dried fruits are Winco's.
Whether on the trail or at home, I avoid eating stuff that humans weren't eating 100 years or more ago. Don't need 47 double-blind studies to know that "foods" like Gu contribute to the astronomical increases in cancer.
Bring a few salt packets, or better yet, a small container of Morton's Lite Salt (half the sodium, twice the potassium) just in case you or someone on the trail needs it.
Listen to your body - it will tell you when to eat/drink. For me, when I start hearing a voice that suggests that maybe this particular hill doesn't have to be climbed today/what are you THINKING ... I know my blood sugar is too low, regardless of whether I "feel" like eating, so just do it. If I still hear that voice 20 minutes later - well, maybe there are other reasons to reconsider, but nearly always I'm recharged.
All things being equal, after a point climbing a big hill is mostly a mind game, so figure out how to short-circuit those negative moments. Keep an eye ahead for particularly steep pitches, and try to eat a high-calorie snack (slice of pinapple, a couple of dates, etc) about 20 minutes before reaching the steep pitch whether you "feel" like it or not, particularly above 12K'. Then you can focus more on your breath, and not be distracted so much by lack of energy.
