Slider for FPS Movement Speed

if (Amount--) and if (Amount++) will always be evaluated as TRUE since Amount is an int (which does not evaluate to 0 or 1) instead of a boolean, so both if blocks are being called when MoveSlider() is called.

Try setting a flag for when your movement is changing to evaluate in your if block. Just to make it easy, I'm using an int in the example below that will always be either 1 or 0.

 var int MoveSliderDown = 0; // starts as equivalent of where Amount-- was
 //..other code declarations...
 
 function OnGUI () {
   //movement speed
 
   if (GUI.Button(Rect(3, 670, 20, 20), "-")) {
       gameObject.GetComponent("CharacterMotor").movement.maxForwardSpeed = 
       gameObject.GetComponent("CharacterMotor").movement.maxForwardSpeed-1;
       gameObject.GetComponent("CharacterMotor").movement.maxSidewaysSpeed = 
       gameObject.GetComponent("CharacterMotor").movement.maxSidewaysSpeed-1;
       gameObject.GetComponent("CharacterMotor").movement.maxBackwardsSpeed = 
       gameObject.GetComponent("CharacterMotor").movement.maxBackwardsSpeed-1;
       Amount--;
       MoveSliderDown = 0;
   }
   //<snip> 
   if (GUI.Button(Rect(133, 670, 20, 20), "+"))
   {
      gameObject.GetComponent("CharacterMotor").movement.maxForwardSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxForwardSpeed+1;
      gameObject.GetComponent("CharacterMotor").movement.maxSidewaysSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxSidewaysSpeed+1;
      gameObject.GetComponent("CharacterMotor").movement.maxBackwardsSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxBackwardsSpeed+1;
      Amount++;
      MoveSliderDown = 1;
 
 }
 
 function MoveSlider() {
 
    if (MoveSliderDown == 0)  {
      gameObject.GetComponent("CharacterMotor").movement.maxForwardSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxForwardSpeed-1;
      gameObject.GetComponent("CharacterMotor").movement.maxSidewaysSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxSidewaysSpeed-1;
      gameObject.GetComponent("CharacterMotor").movement.maxBackwardsSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxBackwardsSpeed-1;
    }
 
    if (MoveSliderDown == 1)  {
      gameObject.GetComponent("CharacterMotor").movement.maxForwardSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxForwardSpeed+1;
      gameObject.GetComponent("CharacterMotor").movement.maxSidewaysSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxSidewaysSpeed+1;
      gameObject.GetComponent("CharacterMotor").movement.maxBackwardsSpeed = 
      gameObject.GetComponent("CharacterMotor").movement.maxBackwardsSpeed+1;
    }
 }

It would be better to use a boolean flag, but my brain's not working correctly right now :)