How can I best showcase my web based service to up to 12000 potential customers as an exhibitor at a trade show before my startup has launched?

Treat the event like a lean startup landing page.

Get people to sign up for a "waiting list" to use your service. Have one or two easily accessible ipads on the side of your booth nearest the walkway, so that people can easily sign up. The ipads should have a nice landing page on it that shows some basic background, a waiting list sign up, and it should say "We're right here to answer any questions!" to draw people in to talk to you. You'll instill confidence in potential customers by how well you present yourself (in conversations with them, in the clothes you wear, etc.) and your booth.

You might use a small subset of your sign ups as beta testers that get to see and fiddle with the website before the official release. Don't use them to look for bugs, but have a questionnaire that they answer about what features they like most, etc.

Have a newsletter (every week or every two weeks) be sent out to all the emails you collect. Each email should highlight one or two features or progress of your site that has just been completed ("we've just partnered with xyz!", "now completed: feature xyz", etc.). This should be done in a sequence and spacing that presents well, and doesn't have to accurately reflect the exact time and sequence of when things were actually completed.

If you'd like finer grained advice based on your actual website and potential customers let me know and I'll see how I can help,

Lee

What Lee suggested is good. Another option is to create a demo animated video and post it on YouTube or Viemo and share it on your landing page.

Lee's answer is great.

What I will add is that in this type of setting you should be able to convey your product/service value add in one to two sentences. Then lots of repetition/reinforcement. Therefore don't stress about short prep time. Add simple visuals if possible and a concise, clear explanation and this will be best.

You are just trying to generate initial interest and word of mouth spread. Details can follow but the main value proposition should ring like a bell - that is what matters.