One question, though: In places like New York and Boston, you only swipe your card once (when you enter the system). In others, such as Washington DC, you do it when entering *and* exiting. Is this how it's done in London? I ask this because I've heard people say things like "always touch out". Well, if you have to be reminded to do that, is it easy to miss? Are there some stations where it doesn't work that way? If so, how can newbies tell what to do?
You have to touch your card both exiting and entering. I've never seen a station without a turnstile that would prevent you from exiting if you don't touch on your way out.
This problem occurs once you get out of the underground enclosed system and into the stations which are served by the underground lines such as Chorleywood or Rickmansworth, which have Oyster readers but no turnstiles.