Whenever you lend money, the best advice is always to make sure you obtain a written note. Not only is it evidence that the money was meant to be a loan and not a gift, but it lets the borrower know that you intend to be repaid. It also allows you to state when you expect repayment, whether or not you expect interest, and what your rights will be if you are not repaid.
You may also want to consider whether or not to take any collateral to protect the repayment. If so, you need to be sure that you are getting a lien on that collateral that will stand up if the loan goes bad. If the lien is not properly protected, you may end up with no collateral at all.
Before you loan to a relative or friend, always consult with an attorney.