If you like to copy and then paste in plain text you may know (for Windows) PureText.
For Mac, I found Flycut, which is much more powerful, but if you don't care about the advanced features you can still use it the simple way.
To paste as plain text, simply copy something, then use the shortcut Command + Shift + V.
If you hold Command + Shift + V, you can navigate with the "left" and "right" arrows your clipboard history. If you don't want that, or you want to customize it (say you want it to remember only the last 5 , check the Flycut preferences. Flycut has an icon in the top bar. Click it and then click on "Preferences".
Remember to add it to your Startup items, else you have to open it manually all the time.