If you don't have secure parking at your workplace, you should indeed have a serious lock, such as a Kryptonite. The thing is, you should not carry it home every night. The weight of a typical U-lock represents the difference between a $400 bike and a $700 bike.
Just leave the lock at work, locked to whatever you normally lock your bike to. Carry a light cable lock with you for quick errands or emergencies. It is easily cut, but you are making a trade-off between security and convenience.
If you use both the U-lock and the cable lock at work, you are more than twice as safe as you would be with either of them alone. Either type of lock can be defeated, but each requires a different large, bulky tool which is useless against the other.
The cable lock will secure your front wheel to the frame and any convenient object, and the U-lock will secure your rear wheel and frame. If you have a quick-release seatpost bolt, replace it with an Allen-head bolt, and stop worrying about having your saddle stolen. [If its a pre-softened Brooks titanium-rail Swallow, put a cheap saddle on your commute bike, and then you won't worry -- John Allen]
The best cable locks are the ones that have the lock built-in, rather than relying on a padlock. The padlock is the weak link, easily cut with bolt cutters, the tool of choice for most bike thieves. A new, sharp bolt cutter will cut a cable too, but an old, worn-out one will only crush a cable.
The best U-locks, if you must carry one on the bike, are the smallest. My favorite is the Kryptonite Mini, which not all bike shops stock. The Mini is much smaller and lighter than the more popular models, but just as secure. It may be even more secure, because of the limited room to put a jack inside it. It also gives less purchase for leverage-based attacks.
People tend to buy the big clunky U-locks because they don't know how to use them properly. A U-lock can go around the rear rim and tire, somewhere inside the rear triangle of the frame. There is no need to loop it around the seat tube as well, because the wheel cannot be pulled through the rear triangle.
Some will object that felons might cut the rear rim and tire to remove the lock. Believe me, this just doesn't happen in the real world. It is indeed possible to cut the rim with a hacksaw, working from the outside to the inside, but first, the tire must be removed or cut through. It would be a lot of work to steal a frame without a usable rear wheel, the most expensive part of a bike after the frame.
[On the other hand, a slightly longer lock would fit around the rim, and the chainstays or seatstays...or locking from the left side might make a little room because the chain doesn't get in the way...you might need a longer lock too, for when you can't get the bike as close to a pole.
Lock to an object which would be difficult to cut or disassemble, and where the bicycle can't be lifted over the top. Good, secure bicycle racks are becoming common these days: "dishdrainer," wheel-bending racks are seen less often. (Lock to the end of a dishdrainer rack if you can. If one is in a secure area, back the bicycle in:: the rear wheel, unlike the front, will hold the bicycle steady. You can lock the rear wheel to the rack for additional security.) A thick sign pole, cast-iron fence, parking meter post etc. is good, as long as locking there is legal: bicycles also may be removed by police and maintenance crews. Check before locking: some posts can be lifted out of the ground, and thieves have been known to cut bicycle racks, taping over the cuts to hide them. A lock which passes around a rim makes the bicycle unrideable even if the object it is locked to can be broken or disassembled.
Also note, a bicycle with a single main tube poses a locking challenge. Some folding bicycles have separable seatstays too, and locking the rear wheel as shown in the photo below will not secure the frame. You need to pay extra attention to be sure that the lock passes through a part that can't be separated. Or take advantage of the bike's folding and take it in with you. Ask your employer to install secure, sheltered bicycle parking at your workplace: a bicycle is as easily lost by sitting out in the weather every day, as by theft.
[Think about what kind of bicycle to ride to where you must lock it up. A bicycle that looks like a junker can ride like a dream. An older frame with scratched and chipped paint is good choice. Equip it with nice but undistinguished-looking components. Let it get dirty. Sheldon's Mead Ranger is a good example. That's the original paint, or what's left of it. This bicycle is really old. (1916! -- maybe too old, it's collectible, but most thieves wouldn't understand such subtleties. In any case, this isn't a daily ride-to-work bike.)
You may be well-advised to lock your bicycle at home, especially if you must leave it in a public area such as an apartment-building basement. Even if this is inside a locked perimeter, dozens, even hundreds of people you don't know -- not only your neighbors but also maintenance crews and who knows who else, will see your bicycle there day after day.
Remove easily-removable items -- lights, bicycle computer, bags. An easy-to-carry bag with a strap makes this more convenient. Some bicycle bags have this feature, or you could use a small backpack.
In a high-crime location, take extra steps to make your bicycle unattractive to thieves. You can easily disable a bicycle so it can't be ridden away. Taking the front wheel with you isn't much trouble if you are going to be at work all day. A folding bicycle offers special opportunities along these lines. The Raleigh Twenty shown below would be very low on the priority list for a thief.
There is no absolute protection against theft. Homeowner's and renter's insurance usually covers bicycles, but it won't pay for the inconvenience of being without a bicycle, or recoup the entire loss. The more expensive bicycles these days are sold on flash and spec hype. Light weight in a bicycle counts for much less than you might think because, after all, you are by far heavier than the bicycle. The modest increase in performance between a $400 bicycle and a $10,000 carbon-fiber wonder is important to racers -- but for your daily transportation, choose a reasonable balance between expense and performance. Also, having more than one bicycle ready to ride is convenient if you discover that the one you rode yesterday has a flat tire, or if one is stolen. -- John Allen]
[My Raleigh Twenty, brush-painted with drippy marine enamel.
I left the bike here two days. I took the seat post and saddle with me, along with
the touring bag. I took the hinge key too. The red arrow points to my
old Kryptonite lock, which secured the front wheel and the frame.
The rear wheel is hard to access, behind the front wheel, and
hard to remove, with its internal-gear hub's nutted axle -- John Allen]