Best way to break up node edit forms smaller, more manageable forms?
I use the field group module to simplify things.  This allows you to easily make collapsible groups of collapsible field sets, use horizontal and other types of tabs, and split a single form into multiple pages.

With the field group module, you can also easily add a css class to any group of fields, which makes it straightforward to theme that specific field (change the background color, increase text size, etc.) for emphasis.