Let's say (hypothetically of course!) that I hadn't done much core work in recent months. Now I have tight spinal erectors and hip flexors, and want to build up my ab strength. By strength, I just mean that I want all my core muscles to be properly balanced. What would be a good program to get started on?
CT's ab articles are a good place to start.
I'd recommend this article by Mike Robertson. It focuses on core training and how to improve posture with it.
Also, make sure that you do hip mobility work every day and static stretch 2-3 times per week.