Shift + CMD + / the only shortcut you need

The only OS X shortcut you need to remember is Shift + CMD + / which is actually quite a claim but it won't do any harm to forget "Option-Control-Command-8" and replace it with this!

This is the universal shortcut to toggle an application's help menu. Not only can you search for application help topics, but menu items are also returned.
Read more about this here!