What is the number one thing a startup founder can do to ensure a seed stage investment?

Have a product that users / customer are using and continue to use. The goal is to derisk the opportunity for investors and the biggest risk is that you'll build something that nobody wants.

Once you have that, then try and find a big enough market and opportunity that justifies a venture investments (many startups don't).

And finally, once you do those 2 things - spend time building a list of investors near you, or that have invested in companies near you, then network your way via the entrepreneurs that raised money from those investors to get introductions.

Nothings guaranteed but those things will definitely help the probability of you closing a seed round.

Traction. Traction of course differs from business to business, but the result from an investors perspective is the same... reduced risk. If you have a b2b saas its customers, preferrably paying. If you have a consumer mobile app its users and engagement, and so on.

DO go to meetings with a clear picture of your traction, metrics, etc. be able to explain and support that data.

DO NOT be wishy washy about your metrics, either know it or wrie it down. Investors respect the cheat sheet.

Finally, don't kid yourself with exaggerated metrics and tracking. Read this from Andre Chen if you arent sure - http://www.slideshare.net/andrew_null/zero-to-traction

Have a customer for your customer who want the product with your stuff in it badly along with that initial prospect. You can demo and pitch and present stuff all you want. If you’ve got that kind of market interest depth and validation, you’ll get money assuming the right projected business model. Realize they can replace you - they can’t replace an invalidated idea.