When fasting (Can't right now, I'm pregnant) I find 36 hours the easiest. I eat dinner Day 1, don't eat Day 2, break the fast with breakfast Day 3. I find fasting this way, even up to every other day, easy and my body quickly adjusts such that I don't feel hungry at all on fasting days. I do find it easiest while eating a mostly whole foods, low carb-high fat diet. High carbs tend to make me super hungry.
My husband on the other hand HATES shorter fasts. He feels hungry on day 1/2 no matter what he does and he feels best and does best with 4-6 day fasts. By day 3 he's in a groove and feels great! This doesn't change for him, eating whole foods and low carb high fat makes fasting easier, but 36 hr fasts are just not for him.
I really love the work of Dr. Jason Fung- if you're interested in some of the science his work is great and there are some amazing youtube lectures.
For me drinking to thirst, making sure I am taking in enough electrolytes and keeping busy are key. I will have a cup of coffee w a T of cream on fasting days. I am interested in trying for a 7 day water and salt ONLY fast after baby and nursing. There is some interesting research about autophagy and cancer that makes me interested in a longer fast. If I were prepping for a 7 day water fast I'd be focusing on really nutrient dense foods and suplimenting electrolytes for a good 2 weeks prior to the fast.