The David Allen Co. has an Implementation Guide that is super helpful, I think. It will take you step-by-step through exactly what you need to do. No other resource does it quite like this and it's very inexpensive.
If your budget allows, consider purchasing the "GTD Live" cds. It's a recorded version of a live seminar.
Like you, I implemented GTD from just the book soon after it first came out. I bought a CD set similar to GTD Live and THAT is when it really clicked for me.
Best of luck!
I am the Party