Fast Break is all about pushing the ball up court before the defense has a chance to set up. If the the defense outnumbers the offense, it is may or may not be wise to attack depending on the situation.
When leading a fast break, know where your teammates are and make the passes early. It might be tempting to telegraph your pass or signal what you want your team to do. But it will result in the other team knowing your intention. So refraining yourself from doing it. If your teammate is tired and nobody is at the up court, don't force it and play it out slowly. Nothing is worse than getting a possession and turning it over almost immediately by forcing a fast break.
If your teammate is leading a fast break, assist the offense by pushing to the other side of the court. Almost all the time, fast break is a match of stamina. And those who can endure the match will come out victorious.