I'd like to see basic trauma life support skills taught as part of race driver licensing and high performance driving school curricula. I'm always amazed when a driver tries to jump out of a crash, pull off his or her helmet, and pretend there's no potential for serious injury. Unfortunately, allowing victims of a major crash to crawl out of a vehicle and move around is rarely the right thing to do. Movement can turn an asymptomatic patient with a spine fracture into a hemi-, para-or quadraplegic. So stabilization of the neck is vital to protect against making a bad situation worse. Displacing a nondisplaced fracture can cause bleeding, nerve injury, and even loss of a limb (e.g. from "compartment syndrome").
TURN THE IGNITION KEY OFF. Even if the motor's not running, the lights, fuel pump etc are powered and can spark, overheat, or otherwise trigger bad things. This is why we all have externally accesible master cut-off switches on our race cars.
Controlling bleeding with direct pressure is important, as is careful observation so the rescue team can be well informed quickly. Keep them talking to you and try to find out what hurts etc. But pulling semiconscious people from inverted wrecks is more for television than real life. Stabilization of spine and any suspect limbs is vital to protect remaining function. The risk of fire and explosion is usually cited as the reason for ignoring this concern. It's a really tough call to decide which is the greater risk in some cases. But explosion and fire are very uncommon in street accidents, and keeping the victims calm, still and stable is most often the better course. This is especially true if the victim complains of ANY neck pain, tingling in arms or legs, or numbness ANYWHERE (especially limbs or urogenital area). In many cases, doing nothing is far preferable to doing anything until the rescue team arrives. Babies are a whole 'nuther story that's beyond the scope of a web posting like this, but the proper seating arrangement saves kid lives (and, as one of my professors told me about two days before my older son broke hs leg at the age of 2, "kids are made out of rubber").
And PLEASE protect yourselves. You can get a small box of rubber gloves in any drugstore. They come in handy when you're in a suit and have to check something under the car, and they protect you against fluid-borne pathogens like hepatitis and HIV. Unfortunately, patients are statistically a much greater risk to caregivers than vice versa regarding transmission of infection. If you choose to help at an accident scene, be careful about blood etc.