I'm super crooked, and my mare and I both have the same dominant side, so I have definitely been down this road.
The first thing to pay attention to is yourself. Where are your seat bones when you're cuing for the right lead canter? Where is your belly button pointing? Is your weight evenly distributed between both seat bones and in both stirrups? Often, we feel like we are straight and centered, when in reality we are crooked -- either from or own conformational flaws, or from a horse's movement putting us in a certain spot (e.g. my mare had a hock injury, and as we were bringing her back into work, I realized that she was pushing me over to the other side so that her injured leg would not have to bear as much load). Think about pulling your outside seat bone back ever so slightly and aiming your belly button at the horse's left ear. (In practical application, you will almost certainly not end up actually twisted toward the outside; this visualization tends to just pull people closer to straight, as the natural tendency for most people is to get crooked to the inside.)
In terms of getting and keeping your mare straight in order to get the correct lead, I think that nine times out of ten a horse resorts to bulging the outside shoulder out as an evasion to picking up the weaker lead. Spend some time really feeling what she does with her body when you ask for that right lead, paying special attention to what happens with her shoulders.
One thing you might try is trotting across the diagonal and then asking for the canter right when you get to the wall. A simple exercise, but it helps take away the opportunity for her to throw that shoulder at you. Timing is crucial, so make sure that you're asking before she has actually made it back to the wall. Once she's on the wall, you've lost your advantage. Another, slightly more difficult exercise that I rather like (but only for horses who are fit enough, as it demands considerable hind end strength) involves trotting across the diagonal to X (so right in the middle) and then turning back toward the wall that you've just left in a 10-meter half-circle, cuing for the canter as soon as you begin your half circle back to the wall. In this exercise, it is of vital importance that you do not try to pull her around the half-circle with your inside rein, as you will cause her to get overbent and essentially force her onto the wrong lead. That being said, if you turn off your outside aids correctly, the somewhat tight turn and the wall right in front of her face will encourage her to step into a nice depart for you on the correct lead. I have found that, with this exercise, even the most crooked, one-sided horses always take the correct lead absent rider error (i.e. trying to pull them around that turn).