How do you build a customer base before you have a product?

How can you build a customer base for your product when all you have is an idea (promise of value)?
I'm thinking about getting my product idea to a couple of forums to engage people in it's design and development hoping that I get to build customers through the process. Is this a good idea? Any other ideas?

What about this approach?:

1. Build a one page website describing your product
2. Add a Form to it where visitors could subscribe to a mailinglist for information. You might include a message field for gathering your visitors opinion about your intended product.
3. Follow up on your list everytime you finish a milestone in your product development.

Posting in forums is a good idea, but I would suggest setting up your site in advance to send people from forums there.

However in forums I would only ask for opinions of the users about your product. Send them to your site, asking if they might have a look and let you know.

Another way might be to search the net about similar products and find out how they are built. You may not need to invent the wheel again. This will cost some time.

Hope this helps.

Something that I think will definitely help you, and you should read about, is customer development. Originally developed by Steve Blank, customer development stemmed from his book, Four Steps to the Epiphany. Here are some interpretations of this theory:

• What is Customer Development? by Lean Startup methodology founder Eric Ries
• Customer Development: How to develop your customers like you develop your product by Nivi at Venture Hacks
• Customer Development Methodology (slides) by Steve Blank

Hope this helps!