Polite "NO"

Sure, it is necessary to be able to say your "NO" to those people who use you though no one says that the "NO" has to be rude and brutal. In case you are afraid of hurting other people and you are afraid of spoiling relationships with them, you can say your polite "NO" to those people. There are some ways to say polite "NO" those who try to use you.
It is not necessary to say "get lost" or something. It will be enough for you to explain people that you have a lot of work to do and so you have no time to do their job. See, the polite "NO" may make them feel the way they have to feel. No one will get offended with polite "NO" as they are people common people of common sense and they realize it themselves.
So, here is the one who came to you with another request and, say, he or she wants you to make a report for him or her. In this case you have to ask a person about those reasons that prevent him or her to make that report him- or herself. People are not ready to be asked about that and so they will barely manage to make up a good reason for it.
So, after the explanation is given it is a high time for you to say your polite "NO". Just explain one that you have a lot of job to do at work and home. Even in case you have no such work you can lie. It is okay to lie about it because those people also lie about reasons that prevent them do their job.