In The Art of War, Sun Tzu talks about two factors that affect whether you'll be successful: Heaven and Ground.
We can think of Heaven as the overall climate or timing of a situation. When Barack Obama first ran for President, he was relatively unknown and inexperienced.
But he judged the timing - Heaven - to be on his side. After 8 years of Bush, America wanted a change, and that's precisely what he represented. The result: he defeated John McCain pretty soundly in the 2008 elections.
Ground, on the other hand, has to do with positioning - how well you are protected against losing. How secure is your position? In 1973, when Richard Nixon resigned from office, Henry Kissinger avoided getting replaced by the next administration because he had entrenched himself in so many different important areas of the department that replacing him would have been too chaotic. He had secured his position there.
As a general rule: because Heaven being on your side is out of your control, your primary focus should be on gaining Ground. Of course, you should focus on Heaven to see if there are any easy opportunities for attack, but your first objective should be to secure whatever position you have.
For example, let's say you are trying to get a raise at work. Instead of simply waiting for an opportunity where it'd be good to ask, your best bet is to secure your position there. Become indispensable. Learn new skills that no one else has.
This takes time, but once you have leverage, you'll be able to ask for more and securely get it.
Of course, if the opportunity arises, you may be able to get away with it, but I wouldn't count on it.