Skin to wood, skin to earth.
My top suggestion: Learn to juggle and then learn to juggle batons. In a few minutes of juggling batons you get the "hand feel" of thousands of throws. Your hands will register nuances of each baton: balance, density, surface. Batons can be juggled in many places that an 8 meter toss can't be made. Take 3 batons with you and get them in the air every chance you get.
Choking up gets more skin on the wood. Skin on wood, then wood on wood: that's why I play the game! Hold it like you love it, cradle it. Don't pinch it like a dirty diaper. This is where juggling gets you somewhere, too.
Get grounded. Play barefoot. Your feet can tell you a lot about the pitch if you let them free. Toe up to the line. Ground yourself.
Throw at more than one kubb from your first stance. Forget 8 meters. 8 meters is overrated. Practice 9 or 10m. I rarely hit kubbs straight on. I practice going side to side, and I know this skill increases accuracy in certain types of wind. I am as accurate at 9m as I am at 8m. I do occasionally knock down 2 baselines when going side to side, a rare side benefit of this cross-pitch baseline throw.
As for rotation, I practice anywhere from 1/2 rotation to 2 and have not noticed a major difference in accuracy (yet). I like being able to adapt to changing weather and pitch conditions, so will continue to vary my rotations.