Write the remaining balance on the gift card at checkout

I have lots of gift cards with store credit, and I tend to use them pretty regularly. In order to keep track of how much is left on a card, I often ask the cashier (at the time of purchase) if they have a price-tag-making machine (those little handheld things), and if they can use it to print out a sticker with my remaining balance. That is the best way to get the remaining balance onto your card, because pen wipes off too easily, and marker can be large and forever. The sticker can be peeled off OR you can write a new balance over the old. I usually also write the date of the balance, so I know when a card was last used. Some gift cards lose balance when they aren't used, so you have to be careful about that.