With the introduction of Presidents' Day last night, I decided that I couldn't pass up this opportunity to discuss when to capitalize president, I mean President.
Generally speaking (as with all grammar rules, of course!), the word president is capitalized when referring to a specific president but not when it is referring to the office in general.
1. The President will be in town next week.
2. President Lincoln spoke first.
Do not capitalize:
1. A president must be thirty-five years of age.
2. The presidents reside in the White House.
The "capitalize when referring to a specific president" guideline is true of the highest official of any land--King, President, Queen, etc. It is not, however, true for other offices, such as senator, mayor, etc.