In Final Draft 12 and 11 you can select text on a script page and drag it to the Beat Board, which will create a new Beat containing this text. Once in the Beat, the text will retain its original formatting.
If you do not see the Beat Board next to your script page, go to View > Split Vertically (or Horizontally), click into one of the two panels and go to View Beat Board.
You can assign a color to "trim bin" Beats (by right-clicking or (on Mac) Control + clicking), and you can group these Beats in a corner of the Beat Board for future reference or use.
To bring the material back to the script, simply drag a Beat to the desired place on the page. It will automatically be converted to properly-formatted text again.
To learn more about the Beat Board, watch this tutorial video:
This functionality is not available in older versions of Final Draft. Information on upgrading to Final Draft 12 is HERE.