How do I become less accommodating to others?

I have been told that I allow people to walk over me, use me, etc. It occurs because I am by nature very accommodating, peace loving, and directly avoid conflict. I get upset (without verbalizing) at the way I’m being treated, but I never know how to fix it.

I’m posting this shortly after realizing it for myself. I’ll answer questions to help you get clarification, so you can help me reshape this attribute of mine.

My goal is to be “less” accommodating; I still want to stay accommodating to a point, but I want more control in my everyday relationships. More control on my intimate relationships would be nice too. :)