Moving to: m/AskMbin!

No bullshit, not a joke. Try masturbating with your off hand.

It is a simple enough task, it gives an (eventual) reward for the work, and it helps build coordination between the muscle groups involved. It's also enough effort that the muscles involved strengthen, and the brain has a chance to build up connections to manage that.

Seriously. It works. I had multiple patients over the years that gained benefits in learning to use their off hand after strokes (heh) or amputation. Same with friends and acquaintances in disability support groups. They'd be struggling, doing their physical and occupational therapy, but eventually run into the horny wall. Things would be difficult with masturbation at first, but it drove improvement in other areas way more than any other task.

Works for men and women (though sitting with a support group slowly working their way around to talking about it was hilarious. The blushing and euphemisms were numerous).

Truth is, anything you'll do often enough that engages all those muscles and the brain will do the same thing. But what tasks are more enjoyable than masturbation? You gonna be chopping onions left handed regularly? Maybe whisking a nice lemon curd daily? Repetition is what builds the kind of dexterity your main hand gets.

A few months of spanking it off handed, and you can try writing, or relearning an instrument. You probably won't ever be as good as with your primary at anything, but you can get good enough to handle daily tasks.