Limit number of devices on any one controller.

  If using USB hard drive - in may be worth giving it its own  controller
    (interface card).

  * Current systems usually give each USB port its own controller. 
    This may not be true of legacy systems.

  While many larger devices don't attempt to draw power from USB, 
    Limit the number of small devices on a single hub.
    Avoid passive (non-powered) hubs.