Creating mockups (also called wireframes) of your ideas will make it easier for clients to understand what you are plan to build for them. Its one of the most important phases of any web project - by making sure the client knows what they'll be getting you have a better chance to really satisfy him and also avoid feature creep.
Fusion Drupal Themes has made a useful template of UI components typical to Drupal for Basamiq, which can help you to make better mockups for your clients. You can find a lot of really useful components at mockupstogo.net.
If you aren't making mockups for your clients yet, you really should start. Mockups should become an important part of the project documentation and can even be used to define project scope in your contracts. If you write text about features, this is always open to interpretation - but with a mockup you hav a clear picture of what you need to make and what the client expects to get.
At TechnoErgonomics, We use Balsamiq for our mockups, its simple, cross-platform (Win / Mac / Linux), and the cartoonish style makes it clear to clients that the mockup is not a final product but a prototype. However, there are a lot of other tools you can use to make mockups, like
