Be Professional, Positive and grateful for the opportunity they have given.
Don’t make the mistake of letting loose any anger, frustration or putting anyone’s nose out of joint when leaving your current place of employment. You can only lose by it down the track either through a less then complimentary reference or further down the track it may hinder future employment chances if you apply for work where a former colleague you were negative too is now in hiring position for the job you want.
Always leave in a positive light, write a professional letter thanking your employer for the opportunity they have given you with a brief reason for deciding to move on.