They need seatbelts...

[StratoNa'vi]

There is nothing to keep a Na'vi down on his or her Banshee or Ikran except for the bond which can be easily disconnected. So i wonder why didn't the wind blow the Na'vi off or why didn't the negative G-forces of diving pull them off their banshees.

[Dreaming Of Pandora]

The Ikran have a queue harness (Eywa te'[insert name] tan'sey mak'ta) which also acts like "handle bars" for the rider. You can see Jake and Neytiri holding onto them in several scenes.

[madman]

The Na'vi have naturally better balance then us puny humans too.

[Dusso Janladde]

Na'vi also have opposable big toes that they use to grab onto the harness. You can easily see Neytiri do this in the scene where we're introduced to Seze. Jake's ikran obviously doesn't have a harness at first, but gains one partway through the flight montage. It's less of an issue with the Toruk, since it's so large, you basically just stand on top.