Matt Griffin outlines a process for responsive comping on A List Apart. This sort of in-browser design calls for using HTML/CSS mockups rather than static file formats (e.g. those produced by Photoshop and the like).
There are a lot of benefits to using HTML in the design process, especially since HTML is the final product anyway. Plus, it is easier to demonstrate the behavior of responsive designs in a browser than it is elsewhere. Griffin hits on another very important point, it is possible to get signoff on these things; having some static artifact isn’t necessary:
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.
As the need for responsive designs becomes more prevalent, our workflows need to adapt. Rethinking and reworking the design process like this not only makes for faster design iterations, but also smoother development later on in the process.