IBM has recently released a new RedBook, called "Security and Compliance Solutions for IBM Informix Dynamic Server"

It looks to be a comprehensive review, covering:

Role separation;
Auditing;
SQL security;
Communication; and
Backup security

It looks well worth a read.

Security is a big issue and I don't find most people take it seriously enough. Have a look at an optimally secure system design, then look at where your environment diverges from that. Break the deficiencies into bite-sized chunks that you can address as and when you have time.