This is my favorite type of tip: the easy fix.
Every swing is different, and trying to get someone to get more upright or come from the inside just may not always work. But stance, grip, ball position - those are easy to change and pretty much universally beneficial.
My biggest light bulb moment was to come up with a routine on the greens. I copied Tiger (because why not?). He had a pretty simple like 15 second routine once he read the putt. Stand next to the ball and take two practice strokes while looking at the hole, address the ball, one more look at the hole, then go. After I started doing that my putting got much better.