If Twitter is good for one thing, it is for promoting whatever it is you have to sell. Some of the best self-promoters out there are rappers, and they've taken to Twitter just like every other type of celebrity. Just as Twitter can drive traffic to Websites, it can also drive music sales on iTunes. On a panel at the 140 Characters Conference yesterday, Xavier Jernigan (@xjernigan, the director of digital marketing at Universal Motown Republic, described how Twitter help put one of his new artists, Asher Roth (@asherroth), on the map. Roth released his first album, Asleep In the Bread Aisle, on iTunes on April 20, a Monday. The night before he Tweeted out to his followers (he currently has 69,566) that the album would be on iTunes. And then he Tweeted out a short link which opened up to the album page in iTunes. With no other marketing, the album rose to become the No. 1 digital album for the week. Since then, it has sold about 100,000 copies.