As far as when people are coming over, I do have notes, as Karen said. I printed a small picture of a kitty and typed on it, "Please don't let the kitty out." - they're covered with clear packing tape, and any time we had people over, I put one on each side of every door to the outside, right at eye level. I don't use them anymore, as everyone is well aware of Pinot's presence, but they did serve as a good reminder the first couple of years.

When it's just you at home, you just have to be vigilant, that's all. Kitties do learn; you have to make it obvious that door darting is not allowed.

Good luck!