Most men's suit jackets have three buttons down the front. Leave them all unbuttoned and you look informal. Button them all and you look like a schoolboy in his first suit or a school uniform. So which should you button and which stay open? This rule is easy to remember: "Sometimes, Always, Never" from top to bottom.
Honestly, this is a rule I've known for a long time, but talking about it with others (and the folks at the Art of Manliness) revealed that it's perhaps not as common knowledge as I thought, and it really should be. They explain:
Starting with the top button and working your way down: it's sometimes appropriate to have the top button buttoned along with the middle one (a stylistic decision -- if the lapel is flat, it can look good to button it; if the lapel rolls over and hides the top button, only button the middle one), it’s always appropriate to have the middle button buttoned (the middle button pulls the jacket together at your natural waist and lets the bottom naturally flare out around your hips), and you should never button the last button (doing so messes up the intended tailoring and flare offered by the middle button).
I'd add that the bottom button runs the risk of drawing the jacket in too close or being too tight around the waist while you move around, and is especially unflattering if you have a bit of a belly under that jacket. It may seem like a silly rule, but there is rationale behind it, both from a fashion and a tailoring perspective. There are exceptions (some people like to button all three in very formal situations, like if you're a banker, lawyer, or going to a funeral) to the rule, of course. If it's all just too complicated for you, you could always just buy two button suits and get around the problem entirely.
The Sometimes, Always Never 3-Button Rule [The Art of Manliness]