You can talk about designs all you want, but ultimately you need to sketch them out to get an idea of what they look like. That was definitely the case with our designs for the deck. Pencil and paper are great, especially when you're trying lots of different ideas. (Actually, I like to use a big fat Sharpie, because that leaves you to focus on the overall shapes and not tiny details.)
But if your drawing skills are limited, like mine, for a complicated drawing you might need a little help. For the deck I used SketchUp, which is a 3D drawing application that's really pretty easy to get the hang of. You don't have to be a pro to use this, it's pretty simple to draw some rectangles, extrude them out into space, and paint some textures like wood or brick or grass onto them. And you end up with something like this:
Which I think is super cool! Check out SketchUp, and there's a great series of tutorials right within the application to get you started.