I was recently asked to think of any idiosyncrasies that I have for a theater workshop so as to ensure I didn't carry them over into a character inadvertently. I found this quite an interesting problem, and at the time couldn't come up with any. Over the next few days however I mentally observed myself and started noticing some. Namely instead of waving to people, I touch my middle and index finger to my forehead then wave them out, half way between a salute and tipping an imaginary hat. I also noticed a tendency, when achieving a small victory (getting an answer right, the bus I'm waiting for arriving and so on) I will say "Bing-bing" like the correct sound effect you often hear, but I say it completely flat and monotone. A more obvious one that I didn't really even think of as something 'me' until someone pointed it out to me is the fact that I almost always have a pair of sunglasses on and unless I'm in the sun they'll be pushed up onto my head to act as a slightly more manly replacement for a headband for any hair that escapes my ponytail.
So what are your distinctive traits or habits? Take some time to think about it, it's pretty much impossible to monitor them while you're actively thinking about them, but with the idea implanted in your subconscious you may start to notice them.