cranberry juice is your friend. It naturally cleans out your body of toxins, so drink a shit ton of it between now and then.
Also, thc is stored in fat cells, so exercise your ass off.
The day of your test, make sure your test piss isn't the first piss of the day, as that is when the most toxins are released. Drink enough water to make your piss run clear. Since it's just your parents giving you the test, you probably won't have to worry about then questioning if you've tampered with it.
If this is going to be a professionaly administered test, stay away from the body cleansing shit you can get from health food stores. Most tests now a days also test for the cleansing product.
'I am going back online. I feel safe there.' - Piot Modrak aka lord_piot