Make sure you don't become part of the problem, first off. By that I mean don't go running out into a freeway and risk getting hurt yourself, or stand behind wrecked cars where another car can crash into the first two and pin you. (that's bad) PingSpike, we park in front of an accident for a few reasons: to make it easier for us to move the gurney and supplies out of the back of the ambulance, and so we stay out of the way of flowing traffic.
If that is all fine and dandy, and you still want to help, there are a few easy steps after calling 911.
If the scene is safe and no threat of any secondary problems (fire, getting hit again, etc) leave the people where they are.
If the scene is only fairly safe, ask the people that are able to walk to get away from their cars and sit nearby. The more seriously injured should not move, and if it can be done they should not move their head unless there is an airway problem.
If the scene is not safe, get everybody out.
Those are only basic guidelines and it changes with EVERY accident scene, trust me.
P.S. Watch out for blood, broken glass, torn metal, and above all SPILLED ANTIFREEZE! (it's slippery like ice)
P.P.S. Carry medical gloves in your car if you are the type of person that may help, if you can't find any medical gloves kitchen rubber gloves should do for temporary protection.