Ray D. Johnson is no ordinary crime prevention expert. He has spent nearly half his 52 years in prison for committing armed robberies, theft, burglary and forgery.

Now Johnson (who is often confused with the "You can call me Ray J. Johnson") earns his living by telling people how they can thwart criminals through his consulting firm, the Dallas-based Crisis, Inc. A robbery-prevention program he helped develop for the 7-Eleven food-store chain has reduced robberies up to 50 percent in some regions, say store officials.

Familiar to regular viewers of "The Tonight Show," the ex-convict says he "goes through clients' homes and businesses and shows them all the places I could get them. Then I tell them how to fix it."

The secret to crime prevention is the knack for thinking like a criminal, says Johnson. "Play cops and robbers" when Burglar-proofing homes before summer vacation.

A week or two before you go away, stand outside your home and look for any ways to get in, he advises. Then fix those things.

"Probably the best deterrent is to get someone to stay in your house while you're gone," he says. "Or get a neigbor to pick your mail and turn lights on and off to give the appearance that someone is there."

Since burgalrs dislike light and noise, Johnson advises investing in an inexpensive burglar alarm and connecting it to your stereo speakers or to outdoor speakers.

Make sure you have a good sturdy wood or metal door and door jamb secured with at least a 1-inch throw dead bolt. "Use the locks," says Johnson, who notes that the front door is unlocked in about 30 percent of residential burglaries and that most are committed bu amateurs, often youngsters.

"Take your priceless things, stuff them in a dirty sock and throw it in the bottom of your laundry hamper," he suggests. "Or put your rings in the ice tray in your freezer and hope you don't have an alcoholic burglar.

"Short of a moat and an alligator," Johson warns that there's no fool-proof way to avoid a burglary while you're vacationing. But the following pre-vacation checklist, gleaned from suggestions of federal and local law enforcement officers, can help prevent a vacation-spoiling burglary. ADVANCE PLANNING

Call the crime-prevention section of your police department and arrange for a security check. Most will send an officer out at your convenience, free of charge, to go through your home and suggest ways to crime-proof it. "Operation-Identification" officers can help you etch your Social Security number onto valuables.

Tell a trusted neighbor where you will be and give him or her a key. Ask the neighbor to check on your home, pick up your mail and newspapers, use your trash cans, occasionally park a car in your driveway and do anything possible to make your home appear lived in.

Do not stop your mail or newspaper deliveries, advises Montgomery County Police Cpl. Howard Miller, who said burglars have been tipped off that a house will be vacant by a "stop notice." (However, stopping the services is better than letting newspapers and mail pile up if you can't get someone to pick them up.)

Ask the police to put you on their security surveillance list while you're away, suggests Metropolitan Police Det. Sally Kirk.

Although some officers say this can be a giveaway that you're goner, Det. Kirk and others say it's worth the risk.

Buy several inexpensive timers. Attach two to lights in different parts of the house and schedule them to on at staggered times. Attach another to a small radio, placed on a hard surface, to switch on for a few hours in the morning and evening.

Don't spread the word that you're vacationing and never put a notice in the paper advertising your trip.

Arrange for someone to cut your grass and trim shrubs if necessary.

Ask a neighbor to use your parking spot, if you live in an apartment, and don't tell the desk help the you will be gone.

Put jewelry and valuables in a safe deposit box.

Post an inexpensive "Beware of Dog" sign.%v. HEN YOU LEAVE

Turn down your phone so a potential burglar standing at the door won't hear unanswered ringing.

Unplug all small electrical appliances, including lamps (except those on timers) and TV sets.

Set your air conditioner for a reasonable temperature. Having it lie idle in 95-degree heat can be a dead giveaway.

Close the fireplace flue; make sure outside water taps are drained; turn off the water to the automatic washer and place shades in normal daytime position.

Take a last walk around the house. Check all windows and doors to see that they're locked. Make sure the front door,the garage and any cars left are locked. CAPTION: Illustration, no caption, by Robin Jareaux - The Washington Post