Simple. Just write the facts on a piece of paper and both you you sign it. That's all that's necessary should you need to sue to get a judgment and then collect what's owed to you.
I, (Your name) have loaned 'x' (their name) $x (Amount loaned).
This is to be repaid to me in full by (date), in the following manner;
(Whatever you two agreed upon) For example; In full by (date), or equal installments on the 10th of the month in the amount of $x until debt is repaid. Or, $x on November 15th, $x on December 15th, and the balance on Feb 1st, 2010.
Then both of you sign the paper. In fact, MAKE THREE COPIES! Give one to the person you loaned the money to. Place one in your dresser drawer, and mail one to yourself or parents and do NOT open the envelope until (if ever) needed in court. That unopened envelope will have a post date on it and be proof you both signed and agreed to the terms at that time.
Next time, don't loan money. ;-)