One more thing: if you use the DLR, make sure you find the little console where you can "touch out" (it's usually on a waist-high console, and you "touch out" by holding the card against the little yellow disk with a card icon on it), thereby registering your exit. There will be a little beep, a green LED light will illuminate, and it will say "EXIT" on the little display.

There are no barriers at any DLR stations and it's very easy to forget to touch out - do NOT make this mistake, or you will be charged for a journey out to Zone 6 (because they don't know where you actually got off and this is the furthest you could conceivably go) which is quite a bit more expensive.

And when you get back on the DLR, make sure you touch back in. The procedure is the same.