In-game Achievements Analyzed

There's a good article on Gamasutra about how to use in-game achievements and their impact on the players.  Achievements are definitely something that I would like to eventually put into my games because there are plenty of cases in which I am going back and trying to find all of the achievements within a game even after I have "finished" a game.  Personally, I hate when there are achievements that can only be earned once so that you can't go back later.  Most of the time I am playing a game I am not focusing on the achievements until I finish the game and it sucks to find out you can't go back to that achievement unless you start the whole game over again.  I would be interested to hear about your ideas of successful achievements.