Take a picture of the person with the item you are borrowing them. Not only will you be able to take a quick look as a reminder to the items you have lent out, but you will be able to prove to them they did borrow it in case they were to forget.
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If you are always forgetting things and need some ways to improve your memory, then take a look at some of these helps. You will find exercises that improve brain function and tricks to help yourself remember more.