Moving in with your significant other has many benefits, but saving the environment isn't one of them. The larger a household, the greater its carbon emissions.
Becoming a green team is essential to keeping your carbon footprint low once you've shacked up. Talk to your partner about how you want to address recycling, food (organics, local, fair-trade etc.) and consumption.
Maybe you've already had this conversation, and that's why you fell in love in the first place. (Being green is hot.) If not, start as you're packing up your things—figure out what's essential and what you can recycle. Think: less stuff, more love.