First, check out r/ISEEprep, a "subreddit" I have created on the Reddit website to easily share free ISEE information.
Working with a private tutor is the very best way to maximize your score, for a variety of reasons:
1) You are given personalized attention, lessons tailored to fit your schedule, and the opportunity to discuss each question in-depth until you are fully satisfied.
Homework and Practice: As a general rule, students should spend at least one hour on homework for every hour they spend with their tutor. The usual homework assignment is 2-3 sections from the book (approximately 1.5 hours), working from the front to the back. Students should time themselves, and mark the question where they run out of time, but continue working past the time limit if necessary. Full practice tests should also be taken periodically, at the discretion of the tutor. The goal is to eventually complete at least 4-5 practice ISEE practice tests. Many of my students have taken upwards of 15 practice ISEEs in total before they took the real thing.