When it comes to planning a technology application, you can't beat low tech. Given that I run a web technology company clients are often surprised that I rarely use my computer when planning a user interface. Instead I opt to create paper prototypes using a pen, paper, scissors and scotch tape. There are a number of reasons why this just works better.
First and foremost, there's nothing faster than sketching things out. While some people may agree that it's fast, they may say it's less efficient because you have to get everything right the first time - however with scissors and scotch tape you can pick and choose what you want, adding and combining elements easily. NOTE: previously I just used scissors and left elements floating so that I could share them between pages, however after a lot of experimenting I've found it's better to fix things in place... lest a brisk wind come along and set you back 10 to 15 minutes.
Outside of speed, very few things are more visceral and easier for a client (or co-worker) to relate to than a paper prototype. Using a pen and paper, you take several things out of the equation that often hang people up. Color is not an issue and neither is font treatment. People who might otherwise feel they can't participate can start weighing in, making them feel like they are more part of the process and helping to gain buy in.