If you're working with vendors, or even just friends and family, make sure they know that sustainability's a priority from the very beginning. If everyone is on the same page from the start, it's easier to expect them to follow through – they may even come up with their own suggestions for making the event greener. Make sure your guests know your goal too.
One way to get the ball rolling in the right direction is to create an environmental impact statement at the start of your project and give everyone involved a copy. An environmental impact statement is a document that outlines the impact of planned actions on the environment. You could describe the typical impact of an event like yours, including energy use, waste production and pollution, and then describe how you plan to make it better. You may decide to focus on waste reduction in one particular area rather than trying to do everything at once. Or you could simply state your overall plans for sustainability so that everyone involved has a common goal when they make decisions about your event.