For SSAT’s: I advise anyone taking them to take them as early as possible. I took mine in January and my results were fine, but it is in your best interest to leave time for retakes if you aren’t satisfied with your first scores. As for studying, I’d recommend the Princeton Review’s book for reading and Kaplan’s book for math. The online SSAT tests are very valuable as well.
Choosing where to apply: Don’t choose schools only for the sake of boarding. Instead, apply to schools where you can genuinely see yourself. While you may want to board with all your heart, applying and attending a school where you don’t fit could leave you miserable. Many people have a ‘cast a wide net’ attitude for all situations, but I personally don’t believe that the attitude is appropriate in every individual circumstance. I initially wanted to apply to five boarding prep schools, but after meeting with reps, looking through booklets, and talking to students, I could only really see myself at two of them. At the same time, be sure to at least check out as many schools as you can to better make decisions like this. I made the mistake of only looking at a few. I didn’t apply to schools where I (now) believe I could have fit well (like Choate, Hotchkiss, and Groton) because I only considered a few. Basically, cast a wide net for your best fits.
Essays: Seriously, take @nico.campbell 's advice and make them original and representative of yourself and your views. Schools want to pick those who they believe to be a good fit, and writing ‘bland’ essays won’t tell them who you are. You want to make sure that schools choose you for YOU. Talk about your future goals, personal beliefs, or interesting experiences (the medical marijuana cat essay seems VERY eye-catching to me!)
Interviews: While it may be difficult for some to keep calm about interviews, just remember that your interviewer is trying to help you. Don’t overthink the questions asked, and answer genuinely. Focus on telling interviewers why and how you will be a good addition to the school, and why you want to go to the school in the first place. If that reason is simply “I want to go to an Ivy,” or “I want to board,” or something along those lines, I’m afraid you may be in the wrong place. Tell your interviewer about what sets the school apart from others for you.
ECs: Make sure your ECs give AO’s an insight into your values and aspirations, but don’t participate for the sake of looking well-rounded. Two friends of mine (one applying to A, another applying to E) started attending student council meetings one month before submitting their applications. Now, I’ve been in school student councils for a good six or seven years, and I’ve never seen either one of them at a single meeting. Neither of them showed much interest during the single meeting they attended that month, either. I’ll take a wild guess and suggest that AO’s will be able to tell if you’ve joined an EC only to look like a more qualified applicant. Instead, pour your heart and dedication into the ECs you already participate in to show your passions.
‘Hooks’: I’ll take a swing and tell you that while being a recruited athlete or being a legacy will get you some attention, the merits of an application will be the reason an AO will give someone the green light. So, if you don’t have what most CC’ers would consider a ‘hook,’ don’t stress. I know many students at prep schools like Exeter, Andover, Choate, and Cate, and many of them don’t have a hook.
Don’t get intimidated by looking at other applicants on CC. Honestly, being the anxious person I am, I spent a while scrolling through other Chance posts, believing myself to be severely under-qualified compared to applicants I read about. This is definitely my worst regret through this process. My friend’s older sister was accepted to Deerfield with a mix of A’s and B’s, a few ECs she cared deeply about, and an interview that gave the family little hope. Stats don’t tell the full story. At the same time, don’t assume you will get into a certain school based on interviews or your stats. My friend’s sister had a 99th SSAT percentile, perfect grades, and a successful interview, but she was rejected from Exeter (she ended up at Andover, which was her second choice, and she’s currently a senior. She ended up loving Andover.) A friend of mine was a triple legacy with great athletic abilities, but he was waitlisted at Deerfield. My Jack Kent Cooke Foundation adviser cautioned my mother about being complacent, and let it slip that many JCKF scholars had been rejected from prestigious prep schools. Likewise, if someone on CC tells you that you have amazing or weak chances, just remember that they are going off stats alone.
Whatever happens, it’s for the best, and you will be FINE. If you don’t get into your dream school, don’t lose hope. Perhaps you weren’t a good fit, or maybe you weren’t what AO’s looked for that particular year. Maybe competition was extremely high that year. You’ll never know. If you are set on boarding and you get into a school that wasn’t your first choice, love the school that loves you. You were selected for a reason. Attend the school where you can explore your interests and aspirations in the best environment.
This above is cumulation of observations from last year and this year… This is me redoing it 
That said, DON’T EVER UNDERESTIMATE YOUR ABILITIES!! I thought I was good at reading. I do awesome in standardized testing. I love books. I’ll be fine…End results? My SSAT reading percentile was my worst and my overall failed to reach a 90. You should also get started as early as possible and try your best to not slack off!! I remember laying around and procrastinating, telling myself: “I’ll study later. I’m good at this.”