Android : Setting Android Activity Orientation and lock it during onStart

I have an activity that I want to lock the orientation of it based on the orientation of the device when the activity starts, I can't seem to figure it out. The code I have now is

protected void onStart() {
    super.onStart();

    int currentOrientation = getResources().getConfiguration().orientation;
    if (currentOrientation == Configuration.ORIENTATION_LANDSCAPE) {
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_SENSOR_LANDSCAPE);
    }
    else {
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_SENSOR_PORTRAIT);
    }
    doStuffBasedOnOrientation();

}

It seems like it would only work for the first onStart then it will stop changing.