How do you keep people honest about which card they chose?
You could also give bad kingdom cards or kingdom cards with on-gain effects that could benefit you.
Multiplayer game; choose IGG, reveal Trader/Watchtower. In a 6-player game that's 4 Curses (and an IGG) for each of your opponents (and either 5 Silvers for you or 5 Curses trashed). The funny thing is, even if they know what you're doing, they won't want to block you, because that would just keep the other players from getting cursed. You can kind of do the same thing with Messenger and cost reducers (although that's a pretty rare set of cards).
Heck, even without Trader/Watchtower, you might still choose IGG if the number of Curses remaining is at least 2 less than the number of players (the player to your left gains IGG, the other players get cursed by it starting from his left, and the Curses run out before they get to you).