The Hack Your Day blog has a great tip for those of us that spend a lot of time in Microsoft Word each day, although it would apply to Open Office users as well:

Around 90% of the time people who write a Word document will format as they go along. This is probably the biggest mistake you can make when creating any document, here’s why, and how to stop doing it.

My thought is that you should usually define the "Structure" of a document as you go along… which items are H2 vs H3, and then clean up the styling later on.

Hack Your Day » How to save time when styling a Word document