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DON’T forget to lock up — it’s a sound practice, yet something many people forget to do, resulting in millions of break-ins nationwide every year, according to the National Crime Prevention Council, a nonprofit organization in Washington.

“I see it all the time,” said Victor Melendez, a locksmith at Umbrella Locksmith in Manhattan. “Even in New York, people tend to think they’re rather safe, so they just pull the door shut behind them and go about their merry way, and then someone walks right in.”

And if you have been robbed once, the likelihood of it happening again increases, said Gregg McCrary, a security consultant in Fredricksburg, Va., and a retired F.B.I. agent. “It’s very odd how crime is distributed,” Mr. McCrary said. “We don’t know why, but if you’ve been burglarized before, then the chances of getting revictimized are higher, and you want to think about upping security a bit.”

The easiest way to deter an intruder is with a deadbolt lock on the door. Expect to spend $40 to $50. Cheaper, mass-marketed deadbolts are easy to force open, said Howard Nevitt, the owner of Ravenna Locksmith in Seattle.

“People assume that the locks sold to the general public are secure,” Mr. Nevitt said. “A lot of times, when the label says ‘fortress’ or ‘faultless’ or ‘defiant,’ they’re easy to pick, and the strength of the bolt isn’t very strong. The bottom line is, you get what you pay for.”

Even for renters, Mr. Melendez noted, a lock is a good long-term investment. “A lot of people don’t realize that if you move, you can take the lock with you,” he said. Store the old lock and its parts in a plastic bag, and reinstall it on your way out.

Mr. Melendez suggested installing locks an inch and a half above the knob, at roughly the door’s center point. Any higher and the door can be forced open by kicking or shouldering it, or using a crow bar.

Defiant Keypad Deadbolt Instructions

For extra protection, add a swinging bar lock like the ones you see in hotel rooms. “The bars are much sturdier than any chain out there — you can’t force them open,” he said.

When you’re shopping for a deadbolt, there are three qualities to look for, according to Gale Johnson, a Chicago locksmith and editor in chief of the trade publication “The Locksmith Ledger.”

First, it should be labeled “Grade 1” or “Grade 2.” (Grade 3 is the weakest and should be avoided, as should the label “professional grade,” which is meaningless, he said.)

Next, make sure it extends at least an inch into the doorjamb. “Some of the less-expensive locks may have a five-eighth of an inch throw,” Mr. Johnson said, which means the door “is far more susceptible to being kicked in.”

Finally, he said, the strike plate — the plate that attaches to the door frame — should say “reinforced.” Ideally, it should be five inches long, with accompanying screws that are at least three inches long.

In New York City apartments, a vertical deadbolt is the most common, said Mr. Melendez, the Manhattan locksmith. “They’re called Segal locks,” he said. “They’re good, but they’re mounted on the surface of the frame and door with screws, so with enough force, either the screws, the lock or the strike will give. The tubular deadbolts — the ones where the deadbolt goes into the door frame — can withstand a lot more impact. You would literally have to destroy the door to get in.”

To see if the lock on your door is secure, try researching the make and model online to determine its grade. And be sure to avoid locks that require a key on both sides, which are a fire hazard.

If you are worried about tenants or contractors and service people copying keys, you may want to install a lock with restricted keys, like those made by Medeco and Mul-T-Lock. They come with identification cards from the manufacturer that must be presented to a locksmith in order to copy keys. Hold onto that card, though, because if it is lost, most manufacturers will not replace it and you will need to install a new lock.

If you lock yourself out repeatedly, consider a keypad-controlled deadbolt that allows you to input a code that releases the lock. Expect to spend upward of $150, and make sure you change the code from the factory default setting.

Or you might want to try CyberLock, a programmable keyhole-free system made by Videx, where the door is unlocked with the touch of a battery-powered electronic key. Though primarily used by businesses (prices start at about $300), it is a great choice for households with rotating residents. Or if you break up with a boyfriend, instead of asking for his key back, you can simply reprogram the lock.

If installing a lock yourself seems too intimidating, you can pay a locksmith $150 to $200 to do it for you, a fee that should include the cost of a quality deadbolt. Proceed with caution, though, as the locksmith industry is rife with scams.

As Mr. Nevitt noted, “There are companies that have 50 or 60 names that all encroach onto someone else’s name. I’m Ravenna Locksmith, and there is Ravenna Anytime. And you’ll get a guy who has been in business for four months, and he’ll sell you a $12 cheap lock, and tell you it’s safe and worth $100.” (A good strategy is to contact a local locksmith who has been in business for decades; look for the store that says it was founded in 1950, with a street address that you’ve actually seen.)

The best lock of all, though, may be the furry one with the key to your heart, Mr. McCrary said. “I have a Labrador — he’s a pretty good deterrent. Typical burglars don’t want to fool with dogs. It’s just too much of a bother. Barking sends them right along to the next house.”

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Questions about home maintenance or décor maybe sent by e-mail to thefix@nytimes.com.