One of the most important thing when i'm programming is something on screen. It's obvious, i know, but i think that is really important.
How many times, while i'm writing code, i closed everything and said "i'm bored"?
So, i think that seeing something moving on screen with ActionScript 3, in my browser is a good point.
Another hint is to keep a small design document. Just a list of feature for current version, and think about one, maybe two version ahead. I think requirements must be small: better to have many versions (from developer point of view) than one big version, where everything will never go into something usable.
So, after this? On one, maybe two versions, i'm ready to put something on the web.. it will be crap.. but i need to get feedback on my work as soon is possibile.