Define and raise exceptions liberally.

It does the job, but this doesn't really feel like it's enough:

  if not playerIsPlayingForwards():
      raise "Player is not playing forwards'

I'm using this structure instead, liberally raising my own verbose exceptions :

  try:
      assert playerIsPlayingForwards():
  except AssertionError:
      raise PlayerIsNotPlayingInExpectedDirectionError('Forwards')

It takes a bit more time, but it's worth it. You get it back in debug time saved. You also get a nice warm fuzzy feeling when one of your explicit exceptions gets raised, highlighting a very specific problem.