In a fantastic A List Apart double bill, Matt Griffin tackles Responsive Comping: Obtaining Signoff with Mockups:
Sending clients in-browser comps is remarkably easy, as it turns out. We just e-mail them a URL. Clients can look at the designs in various browsers and on various devices, resize them, click links and navigation, and check out hover states. Instead of asking our clients to pretend that an image is a website, we show them a website.