Preventive Controls
Preventive controls are the most effective types of internal controls because they are put in place before errors or irregularities occur and are designed to keep these flaws from happening. Examples of preventive controls are: adequate separation of duties (not having the same person both authorize and process transactions), proper authorization of transactions (a supervisor authorizes a purchase by reviewing and approving the purchase request) and adequate documentation and control of assets (when purchases are made, there should be an approved purchase request and an invoice and receiving documents to show delivery of the items).
Read more : http://www.ehow.com/info_8181582_types-internal-controls-accounting.html