Has it ever happened to you that you wanted to use the toilets at, say, work to find it disgustingly dirty, as if a monster used it before and left it in a dirty mess as a punishment for the next users ? Somebody was talking to me about that problem a couple of weeks ago and a solution just occurred to me.
The place should have one designated toilet called "special clean toilet" that can only be accessed using one's access card. This condition ensures that the central computer knows who has accessed the toilet at which time. (If your work doesn't have access cards, then each employee wanting access to that toilet should apply for an access code to be used at the door keypad.)
Then, when a user wanting to use the special clean toilet finds a mess, they should then go directly to the staff to report it and the staff will come and witness that indeed the toilet was left in an unacceptable state, will check the computer records, and permanently disable access of the previous user.
Do this and either people will start cleaning after themselves all the time, or they will lose access to the special clean toilet after the first time they leave a mess.
Natural Selection at play in its purest form!