Prepare for any situation
You can't possibly prepare yourself for all circumstances, since we'd all be pretty screwed if some Leftovers-type stuff went down, but first aid training goes a long, long way. Hit up your local fire department, American Red Cross, or hospital for a basic first aid class, which will help you help others when it matters most. You'll learn how to give non-creepy, hands-only, live-saving CPR that doesn't require you to basically tongue kiss a stranger, plus super useful skills like what to do when someone's choking, and how to stop major bleeding. And you can also learn how to shock someone's heart using an AED machine if they go into cardiac arrest.
Know you don't always have to do something
Even if you're prepared, know your limitations, because sometimes doing very little is the best possible thing. Let's say you come across a car wreck, and someone is badly injured. As Dr. Stanton explains, just being there is good enough: "Sometimes the best thing you can do is call 911 and talk to the person," he said. "Give them comfort." If the person in the crash is already bleeding badly, any stress on top of that will make them "more likely to have complications."
And to further hammer home the point that real life is not like being in a hospital drama on TV, you don't have to be a hero and save everyone. So if you see someone having a seizure, outside of "keeping [the person's] airway open," (a technique you learn in basic first aid!), your job is to sit there and wait for EMS to arrive. "People feel like they have to do something," Dr. Stanton said. "They try to shove stuff in the person's mouth to keep them from breaking teeth, or put their fingers in their mouth to keep them from swallowing their tongue. And then you just end up with two people hurt instead of one."
Prioritize, prioritize, prioritize
When someone comes into the ER with a problem, Dr. Stanton asks himself the same question: "What's going to kill them first?" Probably that hospital food, says every comedian from the '80s. But if that person is not bleeding to death and is breathing fine, then Dr. S has bought himself enough time to figure out what to do next.
By way of example, let's go back to our imaginary person in a car crash: Dr. S says some newbie doctors can get distracted by the victim's nasty-looking sideways ankle and not realize that they're also not breathing. Prioritizing allows you to focus on what's critically important at the moment. You can only do one thing at a time.