"Mange la marde"
I am very nice with people and they the same with me however in the back of your mind you must have a little "mange la marde" in you which in good Quebecois colloquial French is a slightly more vulgar version of "eat shit"The attitude you need to have is confidence coupled with awareness of the possibility you may need to bare your fangs and if you are even looked at the wrong way you should be able to convey a clear message.
Earlier in my life I swallowed a fair bit of the stupidity of society and hence today I am not to be trifled with. Because although you may want to inflict damage on me I am more than capable of returning in kind.
In other words, "mange la marde" :)