A joystick offers the most precise way to control the jetpack. While mouse and keyboard always command maximum rates and velocities, you can use the stick to command intermediate values as well.
The joystick interface is a browser plugin that offers the javascript code of the simulator access to the joystick hardware. You can download the plugin here.
Unfortunately, the joystick plugin does not work with the latest Chrome version (10) and since the Google Earth plugin does not work with Opera and Safari, right now you can use the stick in the jetpack simulator with Firefox and Internet Explorer only.
The forward/backward and the left/right motion is controlled by the primary (tilt) axes of the stick. The yaw rate is proportional to the rotating of the stick about its vertical (rudder) axis and the throttle axis is used to command the rate of climb.
Since you can use mouse, keyboard, and stick at the same time, an unexpected motion of the jetpack is sometimes induced by a non-zero stick command (e.g. if the throttle lever is not in its middle position). Note that the corresponding buttons on the simulator page are activated in case of non-zero stick commands.