Movement is the biggest cause of simulation sickness, especially movement that is counter to how we behave in the real world. Non-forward motion is very common in computer games where the player can move in one direction while looking in another. Eliminating this type of movement may not be possible in all situations, but it should at least be reduced. One suggestion would be to reduce the player's speed greatly when looking in a direction that is greater than 15-25° of their movement vector.