Wednesday, January 14, 2009
When it immediately precedes someone's name, such as President James K. Polk. (The same goes for other titles, such as "Senator," "Congressman," "Dogcatcher," etc.)
Sometimes, though, "president" and other titles are used to describe a person. In those cases, you leave the title uncapitalized. So you would say, Del Morosa, president of Exempli Gratia Enterprises.
Now I don't know if this exception applies to other countries' leaders, but if you're referring specifically to the U.S. presidency, you can pretty much capitalize "President" in any context and get away with it. Nevertheless, it's probably better to follow the rules and capitalize it only when you're referring to a specific president.