At the risk of this sounding like an ad, I'm a longtime DH client and just trying to spread some love about software that I found and am now using a lot. The software is called Balsamiq and it allows you to create mockups, also called prototypes or wireframes so that your clients and other site consumers can see what your site will look like before you code it. It's very useful for anyone here who develops websites or even thick-client software. You can use a limited nagware version online or purchase a version for production use. I'm very happy that I'm using this software with clients now and I highly recommend it. I'll be happy to answer questions as best I can but lots of info is available at their site, including a video, and you can Google for 'balsamiq blog' and see comments from a lot of people.
I am not a representative of the company and get no commissions or referral benefits.