Use mixed case and prefer lower case over title case. e.g. use "This is a heading" not "This Is A Heading" and not "This is a Heading".
Do not add any unnecessary formatting to document formats which allow for endless formatting variations (Microsoft Word, Open Office, etc.). However, do make use of paragraph styled to denote document structure, see below.
Use standard paragraph styles to denote document structure.
For Open Office this means:
Title: The main document title
Heading 1: A first level heading
Heading 2: A second level heading, etc.
Text body: Body text