project mangement skills
communication skills
effective use of quality tools
effective Reportwriting
Positive attitudes (auditor and auditee)
nonevasive techniques
effective teams, consensus
Strong corrective action system and support
Buy in from department managers (get them on the team.....get any naysayer on the team)
Most of those chop up to experience in one way or another, but are key elements for success, and a starting point for building effective auditors