Stop buying your milk in disposable packaging, plastic or even Tetrapack. Glass bottles are much better as they can be reused easily.
Depending on where you live, a lot of companies offer milk delivery services, the old school way. You can get your daily fresh milk, delivered in a glass bottle to your door. Also, leave your empty bottles in front your door and the milkman (or milkwoman) will take them away and reuse them! Easy!