Here is a hint:
<img src="http://img600.imageshack.us/img600/1007/gameindustry.jpg" border="0" />
Gamers don't like it when the game is overly monetized; they want to enjoy the game for their buck and tend to resent when they are being constantly pressured or reminded of payment options.
Some games took the monetization too far, to the point where gamers simply turned away from them in disgust.
It's like those annoying telemarketers, you know. Phone rings, answer it after dashing out of bathroom only to find out that they're offering you a great deal for tupperware. The doorbell rings, rush there and see a suit guy with wide grin and a brand new SuperDuperUltraVacuumBeast that supposedly vacuums whole rooms in a matter of seconds. If you buy one, you get a free set of magic brooms, too, hand signed by J.K.Rowling!
Well, I exagerrated a bit, but you get the idea. Don't force players, don't overdo - just let them enjoy the game. If they do enjoy it, they'll come back for more. <img src="smileys/smiley2.gif" border="0" align="middle" />