As a mobile gamer, I can say (personally) that some in-game adds don't bother me at all. For example: the game Jupiter Jump by Noodlecake Studios only shows ads after the first attempt (full screen) as well as at the main menu along the top. I think this is well-executed, as it doesn't overload the player after every attempt. Vector, by Nikku, is a little encumbering. After every attempt (roughly every minute) a full-screen ad is displayed. This not only annoys the player, but abruptly interrupts the flow and plot of the game.

Placing your ads away from the main focus of the screen creates far less stress for the user, and carefully managing the frequency at which you display ads can also keep your players engaged and entertained while unknowingly supporting the developer.

One thing to avoid at all costs is "pay $2 to get 50 coins!" If the user downloaded the game for free, odds are that he or she will steer clear of paying for anything, unless they become obsessed with the game (Candy Crush Saga). Also avoid using "sign up for Dish to receive 1000 XP" sort of deals. It's completely irrelevant to the user, and encouraging the user to sling his or her credit card information at one random company or another is borderline immoral.