The best thing you can do is shop for auto insurance every year or two. The longer you stay with a particular company the more likely they are to very slowly and carefully hit you with small rate increases until you're paying far more than their competitors will charge you. Insurance companies understand and exploit consumer behavior better than even the best marketers.
Also, I recently tried to compare my current auto policy with a different insurance provider. The new insurance agent told me my rates would go up with them, so I obviously declined to change my auto insurance.
I understand that periodically shopping for better rates is the logical thing to do, but in my auto insurance shopping experience, the numbers all seem arbitrary so shopping is not useful.