II've never heard one clear, comprehensive definition either... IME, it's often used as a catch-all term for desires/behavior that differ from what is percieved to be the norm (i.e. one man and one woman in a monogamous relationship having lots and lots of missionary sex
). I also think it's often used not to talk about a particular behavior and more as a recognition that whatever the persons' behavior/identity might be, it is somehow considered transgressive by some significant percentage of the general population.
So, one person might call themselves queer because he/she is straight but in a poly/open relationship. I've heard people use it to refer to kinks (especially if that kink is associated with a "community," like BDSM, or furries). I have a friend who identified as a lesbian and only dated women for years until she met a straight boy who likes cross dressing and pegging, so she now identifies as queer whenever she needs to self-identify.
As far as clothing/presentation goes, if you just happen to dress in a "masculine" manner because you find tshirts and jeans really comfortable, I personally wouldn't consider you queer. If, however, your clothing and mannerisms were the result of some more conscious decision to subvert traditional gender roles and expectation (cf a recent photography exhibit consisting of before/after photos of straight couples who took one photo wearing their typical clothes and another wearing their partners' clothes), then I suppose it would qualify.
As you said, the term seems to be pretty muddled, which makes sense given my understanding that the term was created to provide a broader term for people who found terms like lesbian gay bisexual and straight to be too rigid.