1. When departing from Istanbul airport, arrive super early. They actually X-ray you before you can get into the building (not just the departure lounge).

2. Book any internal flights from inside Turkey. Otherwise they charge you twice as much. At least, Turkish Airlines does, the bastards. If you're starting your journey outside Turkey, the way round it is to reserve the flight and ask them to let you pay for your ticket at the desk when you arrive. Which works as long as your inbound flight isn't late. If you do this, ask them to set the deadline for turning up and paying closer to the departure time than normal. I think it was normally two hours but I got them down to one, which was crucial.