Ask HN: Best practices for interface mockups?

I'm working on an idea for a website that would communicate schedules between clients and businesses, but I can't seem to communicate my idea for the interface.

Whenever I need to communicate my ideas or expectations I have trouble getting what I have in my head into something I can show developers or potential users beyond a simple wireframe. I've tried good old pen and paper, but it's difficult to get across the feel of a screen interface onto paper, not to mention my drawing skills leave much to be desired. I ran into similar problems trying to sketch things out on Paper for iPad or Ink for iPhone.

I've seen a few interface builders available on the app store (such as Interface HD) but they seem more limiting than helpful.

What are some of your standard practices and tools for trying to get an interface together for a pitch?