back of an envelope and a pencil stub
LOL -- I thought that, but I didn't write it.
More seriously, the answer depends on how geekish you are.
If you usually buy the same things every week, it makes some sense to
keep a file on the computer that you can print for each trip. Probably
all you need is a simple list written in NotePad, one item per line.
Being an übergeek, and because I almost always shop in the same
supermarket, I made a list in a Word document. It's one page
containing a table, 4 columns by about 45 rows, with the items listed
by which store aisle they're in. Each item has a short underline where
I can write the quantity to buy on the current trip.
You could make a similar list in Excel, or even in Access or FoxPro.
Or you could use a pencil...