Three ways:
1) Actually have separate html files for where page-breaks should occur (ie, dedication, intro, chapter1, chapter2, etc... )
2) Put <mbp:pagebreak/> in the code.
3) using CSS classes:
In the HTML file: <div class="break"></div>
In the CSS file:
div.break { page-break-before: always; }
I use method #1 where it makes sense and #3 where it might not. Just realize that regardless of which method you use... it's still going to wind up being converted to <mbp:pagebreak/> in the actual MOBI's markup.
The above advice is for traditional MOBI, and the new KF8 format has not even been considered.