Elegantly simple -- a Post-It shopping list on the handle of your grocery cart.
I recently discovered this standing in line at the checkout during a marathon holiday shopping jaunt. It was definitely a "why didn't I think of that?" moment.
When you're stopping off for four things on the way home from work, the back of an envelope is still just fine, and you'll still want the special features your smartphone grocery list app has. But on a Major Shopping Trip, you can't really use both hands for scrolling while grabbing all the spices for your big baking project, and it's just no fun trying to juggle a list, a pencil, and three cartons of ice cream (especially when you get so frustrated you give up on the list and then get home only to find you've forgotten your Secret Ingredient -- don't ask me how I know).
If your list is especially long, you can do two or three notes across the handle and still have room left to steer. Just make sure to do a different sheet for every store -- you don't want to get to your second or third destination and discover your list is in the cart corral at your previous stop!
What about you? Do you have any secrets for keeping your sanity on those Big Big Big Shopping Days?