The first week following an automobile accident is probably the most important for finding and preserving evidence of the crash and documenting your injuries. You should definitely see a doctor even if you feel fine; sometimes injuries surface days later. The longer you wait, the more likely it is that the insurance company will deny your claim or think that you're not credible.
As soon as you are able to you should return to the scene where the accident occurred. Find anything that might have contributed to the accident and photograph them. Take several photos of the scene at the same time of day and the same day of the week that your accident occurred. This sounds obvious and you may have heard this all before. Trust me, it will make a big difference when you settle the case or go to trial.
As I have stated in previous posts, take photographs of the car. This will show not only how much damage was done, but the force of the collision.
Of course, make sure you get the names, addresses, and phone numbers of any and all witnesses. Contact an attorney immediately, and DON'T SPEAK TO THE OTHER GUY'S INSURANCE COMPANY.
Even if you have a legitimate accident claim, making any of these mistakes could impact your ability to recover a fair settlement.