How do you get money back that people owe you?

Remind them when you see them that they owe you money. Send an email or letter to request the money. If you are out with them, ask them to buy your coffee or something so that you can recoup the money a little at a time. I once had someone owe me a dollar that didn't want to pay me back and one day I asked is she had any stamps. She offered me three stamps and then I said, we're even for that dollar now.