Safety Audits
if you can get hold of the "Operators Flight safety handbook" published by the GAIN organisation in Dec 2001, it contains a suggested and comprehensive safety audit checklist in appendix D.
A google search should throw up some Ms Word or PDF examples of the handbook, which is actually pretty good in itself.
Don't be put off by the vintage (2001), the checklist and its content are still valid today, and you can use it as a basis to develop your own.
How often to audit ? We are audited by IOSA on a bi annual basis, I interleave those audits with our own "in house" safety audits - kind of "howgozit" audit before we get the industrial strength version.
HTH