[basicxman]

If you want it to toggle, (untested)

bool hasJoystickBeenClicked(Joystick &joystick, bool &prev, bool &cur) {
  prev = cur;
  cur  = joystick.GetTrigger();

  // Previous joystick value should be true (pressed trigger) and 
  // current one should be false (released trigger).
  return (prev && !cur) ? true : false;
}

void getMotorSpeed(float &speed, bool half) {
  return (half) ? speed / 2 : speed;
}

void toggleHalfSpeed(bool &half) {
  half = !half;
}

// ...

if (hasJoystickBeenClicked(joystick, prev, cur))
  toggleHalfSpeed(isHalfSpeed);

// ...

motor.Set(getMotorSpeed(speed, isHalfSpeed));

EDIT: More code.