Preformat your document before you start
Format a blank document with all the required formatting to save much time in the end. Also, the format>tab feature can set appropriate, consistent tab stops. Do this for each section before adding content to save time.
Don't copy and paste from another Word Document
This can transfer clashing formatting and mess up your document. It's hard to fix, so prevent it: copy text into a text editor (notepad, etc.) to strip out the formatting. Copy it again there, and paste it into your current document.
Excel data
Pasting from excel to word creates a word table without Excel style. For Excel style, go to Insert>Table>Excel Spreadsheet, and paste data into the spreadsheet that pops up.
Use Reveal Formatting
Things go wrong? Format>reveal formatting shows where you are and the formatting of the current page. Also, view>thumbnails shows where you are in the document.
