I had some excellent shadowing experiences and a couple of rather lousy ones. One thing I learned quick: if you get a bad vibe, pull the plug. Pick up the phone, makes some calls, and go somewhere else. If your objectives are to learn and ideally to obtain a quality letter of rec, recognize early when personalities don't mesh or the environment doesn't suit your needs and bounce. People like and dislike one another for a million different reasons, few of which really matter in the end. Just give 'em a professional "thanks for the opportunity", throw the hours on PTCAS, and move on to the next. Your time is valuable, so don't waste it on a poor experience.
As for making yourself stand out...how do you make yourself stand out now? Be yourself. Use your people skills. And be mindful that it's someone else's workplace, not an endless game of 20 questions.