I have a friend with whom I meet about at least two months or longer. Last week we saw each other, and I promised him to borrow him a book next time we get to see each other.

I wonder what's the most effortless way to keep that commitment: It is not an item I need to review week after week in the weekly review, but if I put it on a list 'things to remember for next time I see him', and as our meetings are usually unexpected, I should remember that my friend belongs to the category of 'there is a list I have to consult before meeting him'. That list sometimes exists, but other times would not be necessary -nothing pending from our previous meeting-, so the habit of checking it is difficult to create.

How would you go about managing a commitment of this kind?