My fast break mentality is to attack until a defender decides to step up and stop me. I mean, I try to make my decision by the time I reach the free throw line so my passing lanes, if I opt to pass, remain open and unclogged (though it doesn't always work that way) but I always have an attack-first mentality.
I look at it like this, if I'm a hypothetical defender getting back on defense during a fast break, what am I thinking? As coaches always say, I should be thinking about stopping the ball. But then, since it's a fast break, I know stopping the ball might just mean I leave a wingman wide open. So maybe I try to anticipate a passing lane, or maybe I sort of stop ball but not really, just in case I'll need to drop back and address a player running the wing.
My point is, defenders often have indecision on a fast break and frequently, there are times where they want to play a modified version of monkey in the middle. And anytime I sense something of that nature - the defender showcasing indecision, just sort of hopping about, not directly stopping ball, or establishing a weak position up around the free throw line or above - I'm thinking attack.
Of course, the higher you get in basketball skill, the more variables that come into play. For instance, high college level players and above will all have wiser fast break tactics. But if you're talking about playing amongst high schoolers, or college rec guys, or just random street folk, you'd be surprised how many of those defenders may look
like they're in position to stop you on a fast break, but really aren't.
Growing up, my biggest fast break issue was thinking that it was absolutely necessary that I get to the middle of the floor (correct) and undoubtedly always pass to a wing filling (not always correct). Always passing (and clearly always looking to pass) removed myself as any sort of threat and made my fast breaks easily defendable. It wasn't until I watched one of my most skilled teammates (a very unselfish one at that) often continue right down the lane with the ball if noone made a super-serious commitment to stop him. It simplified things for me.
Basically, dribble the ball toward the hoop with the idea of scoring. If someone decides to do something about it, explore other options. Ha, so simple, right? I feel like this guy's coach: http://www.youtube.com/watch?v=lEHZJNQ5Y4A