You're doing exactly the right thing.
Everybody has a natural tendency to skew things in one direction or the other, depending on your handedness. It's very hard to train yourself to notice this in your own work, even though it may be obvious to others. The main way is to reverse the image, so that the skew goes in the other direction. Once you see it reversed, you correct for it, re-flip the image, and re-correct (if necessary). Eventually you will become more sensitive and be able to spot leans and asymmetries without flipping, although it can still be useful as a way to get a fresh look at an image.
I have noticed my images are more skewed when Im not really thinking about the 3dimensional form, they end up looking flat and wonky. The obvious solution I suppose would be to try and be more aware of the 3dimensional structure and relative proportions.
Mirrors are great, I don't know what I'd do without them, the mind can sometimes assume something is correct, when infact it isnt.