I found this over at insurance.com which discusses some of the factors insurance companies look at when determining your premium:

Factors you CAN’T change that impact your auto insurance rates:

• Your age
  Dick Clark and Sophia Loren notwithstanding, aging is unavoidable. And while you may be a mature-looking teen or a youthful octogenarian, the oldest and the youngest drivers are far more likely to have accidents.
• Gender
  Whether it’s the mothering instinct or fewer NASCAR fantasies, women statistically make safer drivers.
• Marital Status
  Ok, you can change this, but there have been no reports of people marrying simply to lower their insurance rates.

Factors you CAN change that impact your auto insurance rates:

• Geography
  Where you live matters. For instance, those living in rural America are far less likely to have a collision or a stolen car than those living in a city. But, sometimes even just moving across the street can change your rate.
• Driving violations
  Speeding tickets, running red lights, failure to yield, etc. all count toward your auto insurance rate.
• Your vehicle
  If you must have that cherry red Corvette or the Ferrari Testarossa, be prepared to pay for it. Your insurance premiums will be higher.
• Accident claims
  While you can’t change the past, keeping your slate clean and free of accidents will hold you in better stead than lots of fender benders.
• Credit rating
  That’s right — many insurance companies view having a poor, or even no credit history as suggestive of higher risk.
• Occupation
  A little easier said than done. Believe it or not, insurers have found correlation between your occupation and risk. Makes sense that the pizza delivery guy could be a higher risk!

Other factors that go into determining premiums:

• Miles driven per year
• Distance to work
• Occupation
• Years of driving experience
• Business use of the vehicle
• Whether or not you currently have auto insurance
• Theft protection devices (often results in discounts)
• Multiple cars and drivers (another opportunity for discounts)