Well obviously that would depend on the situation, some methods that I have used in the past, can be as simple as "I have a credit card bill coming up tomorrow that I need to pay". This case however seems to be slightly more complicated. As I personally don't know the person that owes the money I cant really offer a solution. Everyone has their own motivators, whether that be their family, community, reputation etc. If this person holds himself an honest person, simply telling him that I trusted you to pay me back, and now you are breaking that trust, just might be enough, you never know ! And if he really doesn't have the money, maybe work out a payment plan of some sorts. But the main thing in my experience to do, is to communicate effectively and tell him very clearly (no beating around the bush !) that you owe me money, you must pay me back, and make him give you a time that he will give you the money.
Also many people when collecting money from people that owe them, have this guilty feeling as if they should give him a little more time or just ease up on him, basically trusting them to pay back out of goodwill. I'm not saying this is unacceptable, but at a certain point you must put your foot down.