Matt's answer is a good practical approach for learning how to deal with this issue, but I thought it might be helpful to offer a suggestion about why this is happening to you.
I attended a lecture at Temple University on this very topic, and the professor giving the lecture had the theory that not only are politeness and closeness inversely correlated, but also that perception of your Japanese ability was matched to how familiar you are with casual forms of speech.
Those of us learning Japanese are often beaten over the head with the premise that Japan is a society that holds politeness, formality, and ritual in high regard, and so we must conform not only to grammatical rules, but also to strictly defined behaviours. There is, of course, some truths in that.
However, I fear a lot of the time we Japanese learners over compensate by staying polite and/or formal for fear of making a small verbal mistake that will result in everyone in a thirty yard radius committing ritual suicide.
What a Japanese person is trying to tell you by being "less polite" is not that the rules for polite ritual interaction are different from what you thought.
The Japanese person is trying to tell you that you can relax and take on a more friendly tone, as your politeness makes you sound like you're keeping a distance.
It's also an encouragement, because (according to the aforementioned professor's theory) it means they believe you might be holding onto a textbook approach even though your Japanese is good enough to support more natural conversation.
As a result, asking how polite you should be defeats the purpose because the question itself asserts to your listeners that you do not know them well enough to ease into the right mode. If someone asked me in English "Hey man, how rude can I be to you?" I'm not sure how I would answer, but I would know that the person asking doesn't seem to know me.
The key is to let go of your worry that there is a rule book that has a diagram covering each kind of personal and business relationship and what level of formality and politeness to use. And I say that knowing full well there are keigo manuals and books for Japanese learners that make it seem like there are. Those are guides to start the uninitiated.
More importantly, there is no snake pit. In general I think we have to dispell this notion that Japanese are fragile and will shatter at impoliteness. Trust them that they have a feel for the language that is good enough that they can detect that you tried and missed, and did not mean a genuine offense.
It's an art, not a science, and you will be able to roll with it if you go by feel. In English, you know when to say "Excuse me, would you mind terribly if I ask you the time?" and "Dude, what time is it?" The ability to make such distinctions in Japanese, and in fine gradients, comes with experience.