There's a very telling article by Vanity Fair of the effects of stack ranking at Microsoft: How Microsoft Lost its Mojo. If you don't want to read the whole lot, scroll down to "The Bell Curve"
The important thing is not that you must do good work, but that you must be seen to be doing good work. If a colleague in another department asks you for something, you must ensure that their manager knows that you are helping them out.
Under a stack ranking system, every year a significant proportion of the workforce must be given below-average rankings. The aim is to make sure that's not you.
That article goes on to say that the more mercenary approach is not just to be seen to be helpful, but at the same time, you surreptitiously try to sabotage other people's work. If somebody asks you where to find a document, then you make sure that everybody knows that you found the document for them, while secretly failing to tell them that the document is obsolete and has been superseded by another. If this causes their project to be delayed, then their ranking will drop, causing yours to rise.
Edit: The reason why you must be seen to be doing good things is a process called "normalization". Most managers like to think that their teams are really good, and they want their team members to feel appreciated. So managers tend to give their staff "achieved expectations" or "exceeded expectations" ranks. At the end of this process, this leaves far too many people above average and very few below. "Normalization" is the process of downgrading enough people from their manager's initial rating to bring everything into line with the bell curve. That's why you need other managers to know how good you are.