Good start. Now you should look at what values you get from your LDR, and decide at which value you want to move the servo.
What I think you're trying to do is move the servo to either 0 or 180 degrees, depending on the light level. Above a certain level one way, below that the other way. Right?
void loop() {
unsigned int photores=analogRead(A0);
if (photores > 500) moveServoOneWay();
if (photores < 300) moveServoTheOtherWay();
}
Note the difference for the two movements: that's the hysteresis you need to not start moving the servo back and forth when the light level is about at the trigger point.
Another solution for this (especially if you have short flashes of light or short shades passing by) would be to add a timeout: light has to be below/above a certain level for a certain amount of time before the servo reacts.