The most important thing in preparing for a cleanse is just to think about it, to prepare yourself mentally for what might come up when the ultimate comfort, FOOD, is pulled out of the equation. The more time and energy you devote to preparing yourself mentally, the easier it is to stay on track and stay positive during the cleanse. Setting an intention, or a goal with the cleanse that is elevated (even just slightly) from the mundane and obvious “I want to drop lbs” can give the cleanse a greater meaning. Like setting an intention before an asana practice, you can infuse the cleanse with meaning and make it an even more cathartic experience. Rather than focusing strictly on the difficulty, think of the cleanse as an amazing opportunity to reset your digestion and clean out your system and purge from your body all of the mental and emotional heaviness that you no longer need. Be specific, be introspective, a cleanse is also a quiet time to go inside.
Reading books on cleansing or nutrition also help you stay on track because they explain in-depth the many benefits of cleansing to your mind and body. I love Clean by Dr. Allejandro Junger and Thrive by Brendan Brazier. Reading spiritual books helps you to elevate the cleanse. Just remember that any hardships that come up during a cleanse are opportunities to learn, and you can start eating whatever you want soon enough!