Someone once said that no one can take advantage of you without your permission. If you don't want people to take advantage of you, you have to speak up and say no. You can still be nice about it; you're not being mean or selfish to stick up for yourself.
You did not say how old you, this is a tough one. It takes a while to get the courage up not to be such a nice person all the time that people take advantage of you. You can start "small".. Like reply something like-let me think it over-or-can I get back to you-lines that will buy you some time to think about what it is that someone wants from you and whether or not you can help. We all want to be liked and feel our "friends" can count on us, but sooner or later we all ask ourselves the same question-like you-how can some people take me for granted. It does get old! You are probably a real nice person and deserve to be surrounded by people who make your life better as well. Prepare yourself some quick answers to buy yourself time when someone approaches you, think about it and if the answer is no, then say no, if the answer is yes, fine. It is difficult, but follow your instincts and they will help you. It will get easier and you will feel stronger and proud of yourself for doing what YOU think is best.