Having another person besides you pushing your project forward is the biggest motivator - be it a waiting/potential client, a cofounder, website traffic, any external factor that validates your goal. Internal motivation only gets you so far until you start losing steam.

I think the advice to set up adwords and an intro page before a product is ready really helps with this. My main issue with such a strategy is that you end up having to admit that the product is still vaporware and the time to launch may be too long. Probably worth having an early alpha ready prior to any marketing blitz.