Like most hikers I know, the real answer is: Snacks - lots of snacks.....
I use my vacuum sealer to make peanut butter and jelly packets, but many have just as good of luck with peanut M&Ms and chocolate (I use those too, I just like a variety). I also enjoy the recipie I found on here for Super Spackel - it's actually very, very tasty.
Best idea I use is to calculate the calories you are eating for breakfast and for dinner, then plan on adding to it enough snacks for your needed calorie intake. In many cases, 1/2 or more of my calories for the day come from constant snacking to help keep my energy level from crashing.
I don't get caught up with energy gels and whatknot though. I've tried them, some work well but the texture just bothers me - like I dread eating them. The other problem I had with them is some require tons of water to make palletable.
Many energy drinks help on the hydration front, which is important, but aren't really energy since they don't have a lot of calories.