Drink plenty of water, a gallon a day till the test, start with a glass of cranberry juice. A glass or 2 of water at intervals through the day. Do not give a sample of your first urine of the day. Remember that many cold and allergy o.T.C.'s contain ingredients like pseudoephedrine, which can show up as amphetamine's, in case you get a positive, that may buy you some time for a retest. 
