Shannon,
Very interesting. While I am not super deep in the advertising agency world, I have a pretty good idea of what you are going through. I would reference an article I wrote several months back titled, "Marry your Customer," that speaks exactly to the trust and loyalty issue. You can find it by going to my website, www.customerexperiencesinc.com and clicking on the publications tab on the top.
In a nutshell, you have to start by understanding the "Promises" your clients want you to keep. Then you have to keep them, every day, and over time you will build "Trust." Trust then evolves to Loyalty. You are spot on in identifying that without Trust, there is no chance of Loyalty. Thanks for the comments.
Blaine

