If you want to edit fast on any non-linear system then you'll need to know as many keyboard shortcuts as possible. The problem is trying to remember all of them! Here are two solutions that might help.
Another way to remember shortcuts is to install KeyCue, an application that displays all the available shortcuts in a 'HUD' by entering its own shortcut. It works for all OSX apps too, not just FCPX. We have to thank Alex Snelling for sending us this rather useful tip, all the shortcuts onscreen with a shortcut is pretty cool.
There is a free trial version of KeyCue available too.