> You bring the whiteboard to the grocery store with you?

I take a photo with my phone. It's not perfect because to mark things off you'd have to use the photo-editing on the phone which would be rather clunky for that purpose. But it works if you don't mind checking things off in your head.

> How do you keep it from getting erased?

I've got a glass whiteboard and use liquid chalk markers. It's tough to accidentally erase something with those.