I've been coaching baseball now for 24 years (holy shit, that's a long time) and I'm always happy to impart some info to people who are looking to pick my brains. So ask away!
Here's a freebie: You see a lot of pros these days hitting with their back elbow up at right angles to the ground (this has been taught by a few different people over the years). This is bad technique and anyone you're teaching needs to have that fixed. First of all, lifting your elbow creates muscle tension that first has to be released before you can swing the bat -- that costs you time. Second, almost without exception, releasing that tension causes the elbow to drop down to the hitter's side, where it should have been in the first place, and that motion drags the hitter's bat head down with it, causing an uppercut. Uppercuts = lazy fly ball outs and weak ground balls to the pitcher, unless you're Prince Fielder strong and can turn some of those fly balls into home runs (the vast, vast majority of us can't). Better to set up in a relaxed position, with your elbows at your sides, and take a level swing, aiming to hit line drives.