A lot of you may have already seen this video as you own Fab Filter Pro Q, but whether you use that or any other EQ plugin I think this explains the fundamentals of EQing really well.
A great one for beginners.
[url=http://www.youtube.com/watch?v=kSNYBbPAvKE&feature=player_embedded]FabFilter Pro-Q - EQ Tips & Tricks - YouTube
Fab Filter Pro Q - ftw 
Liked that vid.
Those mastering vids are always welcome!