• Keep things as simple as possible, but not simpler (KISS)
• Before doing things twice, think twice (DRY)
• If things start to get too complex, decomposition is needed.
• Divide et impera - If things get too complex, split them up into smaller pieces and code them separately.
• If decomposition does not help, management is needed.
• Keep in mind: There's a beauty in design. If you find something in your code which looks quite ugly, there is something wrong with it. You probably can do it better.

Marc-André Lemburg (EuroPython 2013)