You've always heard the saying "reduce, reuse, recycle" and now you can implement all three while planning your event.
Reduce the amount of packaging materials you use by purchasing in bulk. Keeping track of the number of attendees can also help reduce waste by making sure you don't have more supplies than you need.
Reuse whatever you can, whether it's cups, plates and silverware that guests can keep using or donating leftover materials to schools and community centers. You can even donate leftover food to local soup kitchens, pantries and shelters.
Recyling waste is a must and you'll probably need someone in charge of this. Do recycling bins need to be emptied periodically? Does the waste need to be sorted? Does cardboard need to be flattened? If so, assign a point person. For big events, plan ahead with your local waste haulers to arrange for pickup and to make sure you're meeting their guidelines for what materials they accept.
Just as important is to make sure the recycling efforts -- and what guests are expected to do to contribute -- are very visible. Use color-coded signs (or even pictures) to make recycling fool-proof. Ensure all your recycling areas look the same throughout the venue so they're easy to identify. Remember, the easier recycling is to do, the more likely people are to do it (that goes for vendors as well as guests).