I live in a third-floor walk-up apartment, and it's a total pain carrying a large amount of groceries up and down lots of times, especially in bad weather. Here are two tips to make it easier.
1. Enjoy the exercise!
2. Take an empty laundry basket downstairs and leave it in your car. When you get groceries, put all the bags in the laundry basket -- you'll usually be able to get everything upstairs in one trip.
