"Neither a borrower or a lender be" It's 500 year old advice, but it is still good.
I find that the best way not to be taken advantage of is to not give people any advantage over me. Don't get me wrong, I maintain very friendly relations with people, and give lots of my time and energy to them, but I have boundaries that I will not cross. One of them is setting myself up for abuse.
In that regard, the only thing I trust is my certain knowledge, that, when it comes to money, no one can be trusted.
keep your guard and punch those who have wronged you incessantly when they cross your path. don't give 'em an inch because.. (sigh) you know.. the whole mile thing.