How to get money that someone owes and won't pay?

A so called friend of mine..actually 3 friends owe me a pretty big amount of $. They are acting like I did something wrong to make them not pay. They needed rent money and I helped, needed money for other bills and I helped. How do I get them to pay? Any ideas? Pull at their heart strings? I'm a good person but when you take money= food and things for my home & son I get slightly irratated. Any thoughts? I'm down to be a little vindictive if nessesary! Needless to say..not helping anyone anymore unless I can afford to loose it!