Non printing characters

How do I make a selected bit of text non-printing?

By selecting the part you want to print in your text and then File > Print > Selection.

or

Creating Non-printing Text
To create text that is not to be printed do the following:

  1. Choose Insert - Frame and click OK.
  2. Enter text in the frame and if you want, resize the frame.
  3. Choose Format - Frame/Object, and then click the Options tab.
  4. In the Properties area, clear the Print check box.
  5. Click OK.

“non printing characters” refers to such as paragraph marks, line breaks, tab stops, and spaces.

To access this command:
Choose View - Nonprinting Characters or Ctrl+F10
On Standard bar, click Nonprinting Characters
Note:
When you delete a paragraph mark, the paragraph that is merged takes on the formatting of the paragraph that the cursor is in.
To specify which nonprinting characters are displayed, choose Tools - Options - LibreOffice Writer - Formatting Aids, and then select the options that you want in the Display of area.

I recommend to have a look at the free guides, which you can download from http://www.libreoffice.org/get-help/documentation/