Also I would add that the corp player can extend the life of a trap by a turn if he follows these steps.
Turn 1. Install the trap as your last click in your turn. Act like there are other things you need to get done. With no advancement tokens on it, it might not be as effective, but if the runner runs on it he is still wasting time and money, which is a major win for you.
Turn 2. If the runner hasen't run on it yet, you can advance it. I like to advance mine 3. It's a larger gamble, but it also has a larger payoff. Advancing it to 3 makes the runner think that it is large scoring agenda, which might tempt him more. Also advancing it to three makes whatever the effect is generally more powerful. Finally, it gives you a legit reason not to "score" it this turn, as it is going to be scored next turn, forcing the runner to make one last decision whether to run it or not. If this is near the end of the game you pretty much all but force the runners hand, since he either gets it or you win.
Turn 3: At this point, if the runner hasn't run it, it's trash and time to throw it out and be sad. Although I have managed to trick a couple of unattentive runners into running on an overripe trap server, this is generally the exception, not the rule.