I would suggest answering all questions truthfully and not using anything other than truth to pass your polygraph.  (You should realize I am a polygraph examiner).  Mr. Truth has a good suggestion for using countermeasures, however I simply suggest telling the truth and cooperating with the examiner.  There is a possibility you will not pass, it is about 14% on screening examinations when conducted by qualified persons.  This 14% includes Inconclusive Results. 

Check the examiners credentials before agreeing to be tested, make sure he/she is an APA or AAPP member and has several years of experience without complaints to the BBB or his professional associations. 

Best of luck.