I just wanted to be up-front about the monetization in my first mobile game, Social Sessions, and explain why I went with the method that I did.
Social Sessions is a free-to-download game which allows players to play until they reach "Day 9" (out of 30 days) before they're asked to upgrade (for $1.99) to continue. The first 8 days will probably afford a player several days of occasional play.
I decided against using advertisements because, well, that's basically just going with the annoy-players-until-they-pay-you model. I don't want to annoy my players. I realize that for many developers this is the best option, and it's possible some very understanding players even appreciate this model and understand why it's necessary in many cases, but it wasn't right for my game.
I also didn't want to fill the game with micro-transactions. There are no extras or cheats or premium currencies in the game to purchase. You just pay to unlock the game once, and all 120 puzzles are yours!
The other decision was, okay, I'm going with the "Freemium" model (upgrade to play full game), but when exactly should I ask players to Upgrade? Some suggested that you do it early, as soon as the player is interested in the puzzles. But in my own experience, I always appreciated when Freemium games allowed you a bit more time to enjoy the game. It allows you to really get into it and play it over a few days so that when you get to the Upgrade point, you really feel like you've played a game already, and upgrading for more levels is just icing on the cake (granted, the icing on the cake is 5 times as many levels as the cake in this metaphor). But then, even if you don't upgrade, and you put the game down there, you feel like you were able to play and enjoy a full game, so you are more willing to go tell your friends to go download and try this game, because it's free, and you can actually enjoy the game for a little while totally for free! And who knows, maybe one of your friends will like it enough to want to upgrade? So everyone wins!
And to those who say that games should all just be offered to players for free, well... I'm not even gonna start...