When packing anything in your lunch container that might leak or spill (liquids, etc.), put the items in those plastic bags we all get from the produce section of our local grocery store. On more than one occasion, I have come across a messy disaster in my lunch box due to something spilling or leaking and all but ruining my lunch.
After many times of arriving at work with coffee or juice of some kind all over my important papers, this tip saved a lot of mess.
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I pack a lunch bag 3 days a week. I hate the smell of a lunch bag when I empty it at the end of the day. I squirt it with Purell liquid, and wipe it well with a paper towel.