What a lot of people don't know is that Text Edit and Notepad are meant for programming and because of this there is no formatting tied to the text. This is truly the best way to put your text into the CMS - by copying it from its source, pasting in Text Edit or Notepad, copying that text and then pasting in the CMS.
If you are using a MAC, you'll want to open Text Edit. For a PC, you're going to open Notepad.
Some of you may not be familiar with speed keys - when holding certain keyboard buttons you'll perform an action faster than you normally would.
Select All: MAC = Command + A, PC = Ctrl + A
Copy: MAC = Command + C, PC = Ctrl + C
Swap Programs: MAC = Alt + Tab, PC = Alt + Tab
Paste: MAC = Command + P, PC = Command + P
Using those speed keys, select your text (this will not work correctly if grabbing from a website), copy it and go into one of the program editors and paste it. Reselect it, copy it and go into the CMS and in your content field paste the content and format it from here.
I've been doing it this way because I've noticed that even if it looks correct, if you didn't copy it from a program editor, it can still have added code on the back end which could cause and issue sometime down the road.