In addition to what's been said above, it's a good idea to have a "Start Reading" Guide Item. Some implementations of the Mobi Reader (eg the Kindle) will automatically open the book at this position.
Place an anchor tag in the HTML at the point that you want it to open at:
<A name=start_reading></A>
Then, in the "Guide Items" section of the OPF file, define an item:
<guide>
<reference type="other.ms-firstpage" title="Start Reading" href="book.html%23start_reading"></reference>
</guide>
The book will (depending on the reader) now automatically open at the position of the "start_reading" tag when it's first opened, and you can return to that point using the "Start Reading" item on the navigation menu.