I too find that the most effective strategy to broaden GTD implementation at my office is to be the embodiment of GTD. I've had pretty-good success by waiting for colleagues to come to me with questions. Here's how I pique their curiosity:
-Use the vernacular: "What's the Next Action", "This is a 20,000ft issue", "I've tickled that for the 18th"
-Overtly use your GTD gear. For me its the voice recorder (some people find it a an extra administrative step, I find it to be the fastest capture method bar-none). I purposefully use the recorder to remind myself of a key next-action in front of a stake-holder, and then follow up on it like clockwork. My biggest GTD coaching success story started with a friend who asked me about the recorder after a meeting.
-Become a steel-trap. Once something goes into my system, it never disappears (unless I want it to). It's amazing how many good ideas and casual commitments end up disappearing down some black-hole. With a trusted system these are easily brought back into you FOV at the right moment. People will begin to wonder how you do it. Or even better yet, begin to mention things to you as means of reminding themselves... and the reminder email or call becomes the perfect segue for an introduction to GTD.