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. You get paid for the work that you do and I need and expect to get paid for the work that I do." There's no need to respond with anger or rudeness. Just state your policy the way I stated mine. If you make any exceptions to this policy, your "friends" (who are really your clients) will refer their friends to you so that their friends, also, will take advantage of your generosity (naivete?). Referrals are good, but not all referrals are good referrals.
Also, pay attention to your gut when deciding whether to accept a new client. If you catch the potential client in a lie, if the potential client tries to negotiate your fee, and/or if you sense that something is off, chances are that you're right. Not all people are worthy of being your client and you shouldn't be blinded by your need to earn a living. If you are, I can assure you that you will regret it.
The way that clients without integrity try to get around paying your fee is to claim that they don't like your work. After all, they can't say that you did a good job for them and, then, either refuse to pay you in full, ask for their money back, and/or refuse to pay their credit card company.
As disappointing and aggravating as this kind of behavior is, if you believe that everyone is, basically, good and has integrity, you're either extremely naive or very young.
The bottom line is this: your clients are your clients, no matter how much you like them; some potential clients should not be accepted as clients because they will try to cheat you out of your fee; and ALWAYS listen to your gut about people, no matter how sweet they appear to be.