Three Second Rule
The Art and Zen to Love and War: Episode VIII. Good advice, except for the fact that it's, well, advice. I'm more interested in information (e.g. if you do x, y will happen) than advice (e.g. 'do x'), because I can evaluate the former and usually bristle at the latter.
That said, the Three-Second Rule sounds about right: in Portland last year, breaking with years of tradition, it took me a second and a half to go through the whole process, which is 1) notice that a girl smiled at me; 2) formulate a response (which turned out, to my great surprise, to be 'smile back'); and 3) then talk to the girl. It usually takes about 20 minutes to do step 1, and by that time, it's too late for steps 2 and 3.