This particular bit of advice will probably seem obvious to most of the readers of this blog, but the number of otherwise well-dressed men I see around London with all their suit buttons done up (or, worse, only the bottom button) suggests that the point is still worth making.
You might well argue that surely a suit has buttons on the front because they are designed to be done up, and leaving one undone is purely a style convention that you are free to ignore? Well, not really. Suits, like most clothes, have any number of elements that are essentially functionless and should remain so. The bottom button on your suit has been merely decorative for so long that it is almost certain that the suit will not actually be cut in such a way that it can do up neatly, and this is the real point of this post. Quite apart from any subjective feelings about style conventions, modern suits simply aren't designed to be worn with the bottom button done up.
another site, for which I apologise) demonstrates my point quite clearly. The top button on a two button suit, or the middle button on a three button suit, is positioned at your waist - the area between your ribcage and your hips that is, or ought to be, the narrowest point on your torso. Fastening a button at your waist pinches the suit in slightly here and gives an attractive and neat silhouette. Below this, as you can see in the picture, the edges of the jacket part slightly as the jacket widens over the hips, meaning that the second button is at least an inch from it's buttonhole. Forcing the two together to do up the button would ruck up the material between the first and second button, it would distort the way the bottom of the suit jacket lies, and it would create a second pinch around your hips, ruining the silhouette and making you look 'pear-shaped'.
A small change but one that will make you, at a stroke, better dressed than about 50% of men I see in London.