Ask HN: How to Keep Customer Advisors Engaged Before Product Offering

I'm working on my SaaS product now, recently I had a potential, large organization (10k+ employees) come to me with interest in what I was doing.

I spoke with them on the phone a bit to understand their needs as an organization and see how they currently are resolving (or not resolving) the situation I'm solving.

After about 45 mins on the phone - they were very interested in what I was doing and I offered the opportunity for them to be a customer advisor to my organization and be an early stage customer - they agreed. They were also interested in a pitch of the product to them in early may 2014 - when I am fighting to have an mvp out for them to work with.

My question to you all is - how can I actively keep them engaged over the coming months? Should I pitch a mockup to them say next month? How have you handled this situation?

Worst thing I see is - I talked to them Mid Dec and leave them hanging until May....

Any advice, thoughts, or feedback is very much appreciated.

Thank you.