I am practicing SCA swordfighting and have started to try to train to fight with the sword in my non dominant hand. So far I find retrain my entire body mechanics. What is the best way to develop them? I know that the only way to gain proficiency is practice, my question is: How do I get the most out practice?
The same way you learned on your dominant side.
You didn't just pick up a sword and be a master swordsman on your dominant side. You had to learn the forms and practice them. You had to start slow, with simple motions, before you moved on to more complex combinations.
The same principle applies to your off-hand side. Start with slow, simple motions, and as you gain muscle and coordination, you'll be able to build on what you've learned.
One thing you can do, outside of sword practice, is work to build coordination with your off hand. You can do this simply by using it more for things you'd normally use your main hand for, including things like eating with silverware, doodling (I don't recommend writing seriously, because that's a skill unto itself), and anything else that favors one side over the other. This will help you build coordination that will transfer to your sword practice.
Keep in mind, though, that it may not happen as fast with your off hand as it did with your dominant hand. The coordination that you have with your dominant hand is the culmination of a lifetime of training. Even if you never picked up a sword in your life, you will likely be more coordinated with your dominant hand than with your off hand. This is because you use your main hand all the time, whether you think about it or not. You've spent the past 20, 30, 40 years or more practicing doing things with your main hand, and some of this crosses over into your sword fighting. With your off hand, you don't have that luxury, and so have to start from a point much farther back than you did with your main hand.
However, at the end of the day, the best way to get better at sword fighting is to practice sword fighting.
I've been moving towards using my left hand while fighting sword and buckler as well as having my left hand forward while using a zweihander.
So far I've found that I get better results by slowly moving through the motions, making sure to get everything right, just tapping the punching bag I use as a pell, not striking it. Eventually over the course of a workout, I move to striking harder and harder.