I would'nt say switching car insurance companies every few years is the best thing to do. I've been with State Farm for at least 10 years, and I can't find any insurance company that's cheaper. Not Progressive, Geico, All State, and afew others I can't remember off hand. I pay $548 full coverage every 6 months on my Mach, and for this area, I think that's cheap.
It certainly doesn't hurt to check insurance rates at other companies, but I wouldn't switch for just a few bucks. It would have to be a pretty major savings for me.
What you are seeing is that many companies offer new policy discounts -- which go away after a while.
What you aren't seeing, is that many companies offer discounts for long-time policyholders that are even better.