Thorough overview of best practices in designing achievements. While these mostly happen in games, it’s more and more common to use game dynamics in more traditional apps. If you’re considering this kind of thing, there is a lot to consider:

When players come across poorly designed achievements, the bottom line is that it lessens their psychological investment into the system as a whole. It is impossible for this to happen without also lessening the sense of accomplishment the player feels when he does earn an achievement in this system.