Running a business / being self-employed we all have to deal occassionally with clients who become unresponsive. E.g. you send them a UI mockup asking for feedback and all you get back is silence. You ask about some details about the feature in their application that you're working on and you don't hear from them for a month.
I try to nag them every few days by sending an email or an IM message, but it not always works. Some promise to get back to you, saying something along the lines of "sorry, I've been swamped with work recently, I will get the answers to you today" - and "today" was 5 days ago, a week ago, two weeks ago and so on. And it repeats like that.
Have you worked out some good methods of dealing with such clients? How do I 'force' them to finally move forward, so that I can resume my work on their project?