To get the "conversation" started, start bringing in your own healthy food! We have a lot of in-office chats about "What's for Lunch?" (even starting a Pinterest Board with Pins straight from our kitchen); everyone has very different tastes and preferences, and we love to try new things. Some have noticed a pure love for things like cinnamon, all-natural almnond butter, avocados and fresh tomatoes (to name a few), and now we keep those staples stocked in the "pantry"!
Once people take notice, steer the conversation towards what's currently available in the kitchen & what "healthy subsitutes" could be made. If there are always candy bars, switch to healthier options like fruit & nut bars. Instead of people bringing in desserts, start a rotation for fresh seasonal fruits. In place of the frequent bagels and/or donuts, ask for low-fat yogurt and granola. The key here would be to think of solutions, and create a new norm for "office food"!
Some tips from our lunch table: do a weekly or monthly office Potluck with healthy foods, invest in a corporate CSA account (we have one with Good Life Farm), snack on nut mixes, and/or start a "Salad Friday" (we order from a locally owned Cafe).