Yet many authors don't write down who helped them, and then end up forgetting about people who contributed. You don't want to put "And for those of you who helped and are not mentioned, you know who you are" in your book. How lame is that?
As you are working on your book, write down the names and contact information of the people and organizations that have helped you along the way. All of them, no matter how small the contribution seems at the time. Don't expect to remember everyone you will want to thank later in your acknowledgments -- and you should acknowledge each person that helped you.
Everyone likes to be recognized for making a contribution to something with permanence. Also, if you need to follow up on something later, you'll know who to contact. Finally, most of those in your acknowledgments will be your little marching marketing army because they are in a book!
Now, back to work!