
Cable locks are convenient. You can just hang it on your handlebar, frame, or maybe some part of your seat. They often come with a holder that you can mount to the frame.
An advantage of the coiled-up variety of cable locks is that they automatically assume a compact shape for carrying. But it can be irritating and troublesome to fight the springiness if you must really stretch the coil to lock up your bike. If you have to stretch it enough, it will tend to pull on your bike and make it hit the object you are locking to.
There is a downside to using some of the cables currently being sold. The thinner ones can be cut using bolt cutters, and many thieves know this. If you want to use a cable lock, choose a very thick one with the metal strands woven at angles to each other and perhaps surrounded by some kind of extra armoring material. Also, get the kind with the lock permanently attached (one end of the cable plugs into the lock), as shown here.

If you already own a cable lock, and decide that it's inadequate, you could still use it to lock your bike's seat to the frame, as shown below. Seats are usually not worth much, so a thief might decide it's not worth the trouble to get yours. You could simply leave the cable padlocked like that permanently.

I show a different use below: you could lock the frame and rear wheel with the cable, in addition to your better locking methods. Even if your cable is not very good, it's one extra problem a thief has to spend time and effort solving.

Shown here is one possible locking scenario, where the front wheel and frame are locked to a pole. If the cable is long enough, you can instead pass the cable through the front and rear wheels; when you do this, it should be easy to trap the frame and pole within the cable's loop.

When you insert a wheel into a bike rack, you won't always be able to lock everything with a single cable.