Ever wondered why there's an arrow pointing left, above the right-pointing-arrow on that tab key? My 8 year old self did. Noticing that holding shift allows you to access the upper character on keys that have two characters, I pressed shift tab, and the focus, it moved backwards! I laughed, as once again the ingenuity of the computer engineers amazed me.
The somewhat unknown big brother to the tab key -- which allows you to move to the next control element in GUIs, pressing shift tab allows you to move to the previous control element. Aha. Logical, one might say, but sadly they only say that after they become aware of the existence of this often-needed but mostly unknown function.
So spread the word folks, tell your friends not to grab the mouse when they want to go backwards, press shift tab instead!