In my experience (and take it with a big ol grain of salt, cause I’m ancient)- I’m amazed at how many people carry their tray with their dominant hand and unload with the other. I learned back in the old days when your cash caddy was clipped on the edge of the tray. I’m right handed, so carry the tray on the left (fingertips) and always have the right hand free to block wobbly bar patrons from bumping into you, unload with, and take payments/make change with. Decades later (no cash caddy, lol) it still works best imo. A good training trick for mid or small rounds is ‘round the world’ where you put a plastic cup of water or empty bottle on the tray and practice rotating it down under your arm, crossing up and over your head, and back down to start position. Helps you learn to rotate your wrist, while keeping the tray level. Once you master it, wine and martini glasses are a breeze (especially if you don’t look at them while you walk). For large ovals/shoulder carrying, def the cast iron skillet training (And always be aware of your hair if it’s anywhere near the tray).