Even just a friendly comment from a new user - enough to know they're not a spam bot, really - would get a promotion from me. I can't remember what the limits are, though - how many people can you promote, again?
That's essentially my rule as thumb, alongside some personal grammar peeves.
This was thrown around, but I've promoted a couple in one day.
Just scroll through there from time to time since that's seemingly the only place for reliable new traffic. Looking into their profile is how I've gone so far. Also, there are people who have been on Hubski more than a year, yet haven't reached a full spoke yet - meaning they can't post. Sometimes it's worth looking.