How does one stop from getting taken advantage of?

I know it's hard. You want to trust people but at the same time they turn around and take advantage of you. Try and remember to say no. It's hard but you need to think of yourself first. Go by your insticts. If it doesn't feel right then don't trust it. Walk away from it. If it's with friends or family, which I know makes it even harder, you need to think it over and tell them the truth. Sorry I can't do that or I don't have the time. Be firm and don't let them play the guilt card on you. If they start stop them and tell them, don't make me feel guilty. I'm not able to do it right now. I know it's hard to be rude but that is what you have to be sometimes in order to protect yourself.