The thing about being nice is that everyone isnt deserving of that treatment from you, but I guess there is only one way to find that out (Im assuming we're not just talking about interactions with strangers). I am just nice & helpful to people - so I cant really get mad at people taking advantage of that. It is what it is. However, I think its important to show someone that you are not easily led the minute they do something you dont like, SPEAK UP FOR YOURSELF. If you dont like the way they respond to your kindness or helpfulness then thats when you fall back.
If youre repeatedly doing stuff for people who continuously show you they arent worth it, thats on you.