How can you be nice without people taking advantage of you?

Somebody please help me out here.

I feel like people take advantage of me all the time because I'm a nice person. I've lived with it all my life, and I'm getting sick of it. I don't want to be a mean person... I just need to find the balance of being nice... and standing up for myself.

If you have gone through this, or have any suggestions for me... please help.

I could really use your advice.

thank you very much.