At a bare minimum, an IOU should include the borrower's name, the lender's name, the amount of the debt, the current date, the date the debt is due, and the borrower's signature. In addition, it's recommended that IOUs contain: How the debt is to be repaid (lump sum or installments)
Outline how you will handle disagreements or misunderstandings. Agree to celebrate each other's achievements and special occasions. Specify if this is a temporary agreement or ongoing. Include a clause that allows for changes to the contract as the friendship evolves.