In the official
Mobipocket site you can find which tags are supported and some further advice (in the menu on the left side). Pagebreaks can be found in the "Mobipocket custom tags" section: <mbp:pagebreak/>.
Note that, in general, you want to keep the HTML as simple as possible, in particular don't use any CSS, that's not supported, and avoid nesting elements. For specifying vertical separation above an element, use the "height" attribute.