I'll put a brief breakdown of Lisa's tips here, but you should read the article linked above for the full run-down.
1. Support your community, buy local pumpkins
2. Walk, don't drive, stick to the neighbourhood
3. Use a costume you already have, or borrow
4. Or make your own costume (no face paints)
5. Use old cans with candles for cheap lighting
6. Re-think the treats
7. Use a re-usable bag
8. Use a solar or kinetic flashlight
9. No phantom power (appliances in standby)
10. Reduce, reuse and recycle!
So with a little bit of thought this Halloween, you can be environmentally friendly too.