For us (KickoffLabs) the lesson we learned early was to engage with (potential) customers on existing own sites and communities.
Too many startups focus on direct advertising and writing blog posts (we we did as well). They forget to add comments to relevant blog posts, other Facebook pages, etc (for us the biggest honeypot of customers has been quora).
Long term, this may not directly scale, but the added benefit is it will yield new customers month after month because of the long term Google effect.