Is it a static model, or an animated one? If there are animations, you need to make sure that the pose at frame 0 is the base/reference pose (i usually add keyframes at frame 0). All animations also need to be in a single timeline with timeline markers denoting the start and end of each (see tutorial). Ideally, you want your model to face the forward direction (press numpad 1 to view front view), but you can also specify a front and up axis along with a global scale in the export settings. You might also want to add event objects for sounds, footprints, etc. (press space and search for "add event object").