1. Do "print at home" tickets instead of having them sent to you. This saves the gas related to shipping.
2. Slather on some chemical-free sunscreen, like Burt's Bees.
3. If you aren't really excited about Fenway food, eat before the game to avoid the waste of paper, trays, and ketchup packets.
4. Bring layers and rain gear. No need to buy one of those silly plastic rain ponchos if the skies open up. Come prepared instead.
5. Walk or take public transit to the game. The Kenmore stop on the green line is super-convenient, so save some money on cab fare and just take the T. If you're worried about crowds, leave before the last inning, or grab a beer at Cask'n'Flagon after the game to wait til the post-game rush to the T dies down.
6. Recycle as much as possible. Don't throw coke bottles or empty beer cups in the trash! There ARE recycling bins at Fenway, and for many games there are also Poland Springs Green Team volunteers who will come around and collect your recyclables.
7. Bring a PeopleTowel or reusable hand towel so you don't need to waste paper napkins.
8. Avoid free stickers, posters, programs, or anything that you will probably throw away after one night.