Ride the fame wave
If all else fails, you can always try piggybacking on someone else's fame - keep an eye on the top Twitter users and pay attention to what they tweet. Reply with something interesting enough and there's a good chance they'll give you a retweet - leading to an instant follower bonanza. But just remember there's a fine line between 'fan' and 'stalker' - replying to their tweet about the state of the economy is fine; tweeting a photo of their house taken from their garden bushes is a step too far.