"How do you keep a laptop computer from getting stolen?" a veteran cop asked with obvious amusement. "Keep it on your lap!" Sounds kind of silly, but it's sound advice.

Here's a simple economics formula: Demand for laptops increases, sales for laptops rise and so do opportunities to steal and resell them. The San Francisco-based Computer Security Institute estimated that laptop thefts topped 250,000 in 1997, but no one knows exactly how many laptops have gone missing -- many victims realize, quite rightly in many cases, that their investment is gone with basically no hope of return.

If you want to keep your laptop, you should follow a few simple rules and use a lot of common sense. Treat your computer like you're walking around with a few grand in cash. And if your laptop is loaded with any kind of sensitive information, treat it like a briefcase filled with those same documents. It pays to be paranoid.

But before you spend lots on complicated anti-theft gadgets akin to what you might have installed in a car, prevention starts with plain old common sense: Thieves can't steal what they can't get. If you can't keep a constant eye on your laptop, invest in a locking system that will keep it where you leave it. Several manufacturers sell leashes similar to bicycle locks for about $20 that allow you to fasten your laptop to a solid object. Other devices, such as Port's Defcon 1 (http://www.port.com, 800/242-3133; about $50) connect your computer to a motion detector that sounds an ear-splitting alarm if someone tries to make off with your laptop. For $130, Port sells the Defcon 3 laptop carrying case, equipped with a keychain-style remote.

You might want to think of supplementing your security hardware with some stealthy software. Manufacturers have devised a variety of approaches to help get your laptop back home if it does fall prey to a thief. Think of it as Lo-Jack for laptops.

CompuTrace from Absolute Software Corp. (http://www.computrace.com, 800/466-7636) silently calls a monitoring center at random intervals. When you report your laptop stolen, the computer center gathers information from the next call and helps authorities zero in on the bad guy. CyberAngel by Computer Sentry Software (http://sentryinc.com, 800/501-4344) detects unauthorized access to your computer, seizes its communications ports and, in the event your computer has been stolen, calls its monitoring center and uses caller ID information to help cops find the laptop.

Software doesn't have to be that elaborate. Laptop Labeller, antitheft shareware for Windows , promises to keep your laptop secure by writing your name, telephone number or any other text you choose into the boot sequence before the operating system loads. It won't keep your computer from getting stolen, but it makes it more identifiable in the event it's recovered by authorities. The software makers claim your identifying information can't be erased from your hard drive.

Simple encryption can keep your important business documents from falling into the hands of people who shouldn't see them. Regular backups ensure that information you need will be around even if your computer isn't.

Then there's the other kind of insurance. Check and see if your laptop can be included under your homeowner's policy; if not, you might check into getting your laptop insured. A Columbus, Ohio, company called Safeware (http://www.safeware.com, 800/800-1492) offers policies for about $50-$70 a year, which also protects your computer against various other types of disaster.

Say your laptop has already taken wing. Realistically, you'll never see it again. But you can increase your odds by registering the serial number at http://www.nacomex.com. The database allows reputable resellers and law enforcement officials to make sure someone's too-good-to-be-true deal didn't come at your expense.