The header should contain buttons that execute actions on the object the user currently sees. Use the btn-primary class to make buttons visually stand out (that's purple at the time of writing), which is a good way to guide the user regarding which would be the most logical action to execute at the moment. Try to have all the highlighted buttons to the left of the non-highlighted buttons and hide the buttons not relevant in the current state (if applicable). If the model has a state, show it in the header using the statusbar widget. This will be rendered right aligned in the header.

The sheet element is rendered as a stylized sheet, and the most important fields should be the first thing the user sees when glancing at it. Use the oe_title and oe_left classes to have them rendered in a prominent place (floating left with slightly adjusted font sizes at the time of writing).

If there are other records of interest concerning the record the user currently sees (such as the partner's invoices on a partner form), put them in an element with the oe_right and oe_button_box classes; this aligns the buttons in it to the right. On the buttons themselves, use the oe_stat_button class to enforce a uniform rendering of the buttons. It's also customary to assign a font awesome class for an icon using the icon attribute.