When at the store, I just it down in a notebook I bought at the dollar store.
When I get home, I put the information into a spreadsheet.
The information I write down includes:
The store name
The item name
The item size
The date price was seen
The price
Was it regular price
Was it sale price
Was coupon involved for lower price
I only have a few places I shop at, so I can usually update the spreadsheet as I shop. I only keep track of the prices for items that I regularly purchase; like coffee, peanut butter, sugar, flour, grains.
edited for clarity