Cut the price, for starters. Originally listed at US$9.99, SGN shot off 80% of that (i.e. lowered the price to US$1.99).
The result? In the first six weeks it was available, according to TechCrunch, "[it] pulled in over $1 million in download fees." Not bad for a company who started out creating game apps for Facebook.
There's no question that a game with a lower price tag is going to be more attractive to the average iPhoner, but you also have to remember: this isn't the work of some guy in his bedroom coding evenings and weekends, this is the result of a team of coders and a serious chunk of investment capital. With all the cheap and/or free "fritterware" clogging the aisles of the App Store, the trick is to convince users that an app that costs more is actually worth more.