I've been reminded in the past couple of weeks how important it is to always follow up with potential clients during your job search.
There was a job listed on Craigslist a little while back that I knew I'd be perfect for. I got together a list of references and emailed the editor. Within a couple of days, I received a response saying they were interested, and would be emailing me the following week to schedule a phone interview.
I didn't hear from them, so the week after that I sent a follow-up email stating that I was still interested, and asking if they still would like to set up a phone interview. A few days later, I had one, and I just yesterday finished my first article for the magazine.
I've had other instances like that recently too. I think it goes to show that it's always a good idea to follow up if an editor or a potential client shows interest, and then disappears. I think they get tons of responses from an ad, and don't always remember which writers they originally showed interest in.
I'm not so sure you should follow up on every job you apply for — just the ones who show interest in your application. As many responses as these folks get, a lot of unsolicited follow-ups would probably get you on their sh!t lists. However, if you are exceptionally qualified for a job, it might be worth sending an unsolicited follow-up email.