It has, recently, come to my attention that people will try to take advantage of your good nature if you let them. That's why you have to know how to say "No".
It's good to keep in mind that no matter how friendly you might get with some clients or no matter how much you like them, they are, first and foremost, your clients. If you view them as friends, first, and clients, secondarily, there will come a time when they'll ask you to do complimentary work for them. This is when you need to put your foot down and say, "I'm sorry, but I don't do complimentary work for anyone.