Is it advisable to follow up on a submitted resume more than once?

I applied for a job at another company 2 weeks ago. A week later I sent a follow-up e-mail expressing my continued interest in the position, and they sent me an acknowledgement e-mail back - a "Thank you for your application, we are reviewing them" kind of thing. Now it's another week later...should I follow up again, or is that considered pestering? If I do follow up, what should I say this time?