Bring along your own carrier bags to the supermarket instead of using the supermarket’s plastic bags.
If you have to use plastic carrier bags from a supermarket on a rare occasion, take charge of the packing of the bag yourself and insist on minimal bags being used.
When buying food to take home, refuse the disposable utensils. You have your own at home.
Avoid using condiments like sauces in small plastic sachets. Frequent restaurants that serve their condiments in glass bottles.