Reducing the number of DOFs, as described above, is another way to simplify 3D UIs. For instance, travel techniques can require only two DOFs if terrain following is enabled.
Finally, when using physical buttons or gestures to map to commands/functions, avoid the tendency to add another button or gesture for each new command. Users typically can't remember a large number of gestures, and remembering the mapping between buttons and functions becomes difficult after only 2-3 buttons are used.