I've been to a couple of them myself and I've learned a few things that we can do to make the best of out a sale. To be honest, I'm still new at this. I'm just sharing some ideas that have worked for me and some people I know. And you're also welcome to share yours via comments.
What kind of book sale is it? Textbooks? English books? Used books? Old magazines? Save your fuel, energy and time by going to sales that offer the kind of books you're looking for.
Make a list. Making a list is a good way to avoid overspending and overbuying, especially if the prices offered are unbelievably low. Unless you're buying books to decorate your shelf.
You don't need to limit your list to titles only. Your list could be of books by certain author or of a genre or interest. A little flexibility here can go a long way.
Check the database/online catalogue on the organiser's website, if there's any. See how many books in your list that you are going to be available for the sale. And if you really, really need a certain book and worry that others will buy it first, try calling the organiser and see if they're willing to save you a copy.
Read reader's review/recommendation on websites and blogs. The power of word-of-mouth marketing. But like all kinds of opinion, be cautious and form your own conclusion.
What kind of book is it? In US books, on the copyright page, there's the Library of Congress Cataloging-in-Publication Data. This data helps clue us in on the book's content. For example, The Fifth Discipline: The Art and Practice of The Learning Organization is catalogued under 1. Organizational effectiveness and 2. Work groups. For Bahasa Melayu books, look for Data Pengkatalogan-dalam-Penerbitan Perpustakaan Negara Malaysia.
Bring a willing and helpful companion with you. Like the list, this person can help you keep yourself grounded, offer advice, keep an eye on the time spent and so on. I stress 'willing and helpful' here for obvious reasons.
Preview read. Don't just grab a book you that heard is good and toss it in your basket. Read a few pages from random parts of the book. Ask yourself honestly if the book interests you.
Pray and hope. Especially when you're really, really looking for that certain book. Keep a positive outlook. Even if you missed the book or even the sale. The world will eventually end, but not because of this.