Appoint employees or volunteers to work as a "green team," overseeing your event's sustainable practices.
Have people near recycling/compost bins to help people dispose of items properly.
Consider setting up an "eco-kiosk" to educate event-goers on how you're greening the event, or set up fund raising for local ecological causes.
Have participants sign up or purchase tickets on-line to cut down on paper waste.
Choose a location that's close to public transport and have transit info available on your Web site.
Encourage attendees to carpool - have Spaceshare set up carpooling for your event!