Well, this is what we are doing:
U.S. commanders have made deals with Shia militias to help provide security for Shia neighborhoods. The Sunni terrorists now concentrate in killing as many Shia civilians as possible, rather than risk losing suicide bombers trying to attack military or government targets. Similar deals have been made with Sunni Arab tribal leaders, to try and shut down the suicide bomb factories and safe houses in Sunni Arab neighborhoods.
Not that many will count these guys when we add up the numbers of security personnel fighting the enemy. For those who say we don't have enough troops, these locally effective security forces will just be more dark matter that doesn't count.