In my opinion, the current rate of progress for PBEMs leaves a lot to be desired. Most games have stalled and Imperialism is taking a month to go through a turn. I will admit that it is disappointing to take over a nation in the middle of a game (and familiarize myself with the scenario and the current situation) only to have the game stop anyway.
I would like to suggest the following incentive scheme. Questions, suggestions and other proposals are encouraged.
This system assumes that 60 hours (2 days and 12 hours) is a reasonable length of time to play a PBEM turn. Players get points for playing quickly and lose points for taking a long time, and should post their running total when they play a turn.
A player gets +1 point if he completes his turn less than 36 hours (1 day and 12 hours) after the previous player, +0 points for playing within 60 hours, -1 point for playing within 84 hours (3 days and 12 hours), -2 points for 108 hours (4 days and 12 hours), and so on. Basically, lose 1 point per day, but the first 12 hours are free.
A player gets +3 points if he allows the next player to use CTRL+N for instead of taking his turn -- but only if he announces it before his turn arrives.
A player can trade 10 points for a reward in game. The reward would depend on the scenario, but I would suggest he get a free turn of production.
If a player falls to -10 points, the next player has permission to use CTRL+N for the turn (this will still give the player +3 points).
If a player reaches -12 points, it is the next player's turn (if he doesn't use CTRL+N, he will start losing points).