Want to know one of the surest ways to save money on insurance? Here it is. Get ready.

Buy more than one policy from the same carrier.

Simple, isn’t it?

This isn’t a state secret. In fact, I just spent a few minutes bouncing around various Web sites, and all of them I checked–Allstate, Farmers, State Farm, Safeco, Nationwide–state in very plain language that they will often charge you less in premiums if you buy a couple of policies.

I’m no student of the dismal science, but this is one economics lesson I do understand. The multi-line discount is, to use one of those puffed up terms economists use, a non-zero-sum game. In English: it’s a win-win deal. Insurers win because they get more of your money in the form of two or more policy premiums. You win because you save money on those premiums. We’re all happy.

According to Allstate’s Web site, a home-auto combo can potentially save you 15 percent, depending on where you live and what kind of home you have. Farmers says an auto-home-life combo could save you as much as 17 percent on your auto premium and 19 percent on your homeowners. Safeco and Nationwide don’t provide numbers, but they both say they’ll cut you a deal when you go for two or more policies.

Multi-line policy offers another advantage–simplicity. Think: fewer bills and e-mails clutter to your already chaotic life. Lots of people pay more, not less, to make things easier.

One caveat: there may be rare situations where you won’t save money by combining policies. Maybe you’re already getting a killer deal on your auto insurance and you should just let it ride. Maybe your carrier doesn’t offer that attractive a multiple policy discount.

For most of us, however, buying two or more policies from the same carrier makes good financial sense.