This video from the Canon Learning Center and stillmotion takes a look at some some basic ideas for holding focus, some tips and tricks for shooting in the field, and devices you can use to assist with focus.
Good cinema should always be three-dimensional - but good sound and good cinematography can make a film 3D without the silly glasses and the headaches.
Eve (Realm DOP) talks us through her top 6 tips for quickly adding depth to your shots, and separating the foreground and background!