It is often stated that the caliber of a man, can be determined very easily by whether or not he keeps his promises. Does this man follow through with what he said he was going to do? Or is he the type whom you can depend on never keeping his word? The Evil Overlord needs to be the first type. This is not so that people will know he is honest. This is so that people know, without any doubt, that what he says he will do, he will indeed do.
For most people, when they threaten to destroy the world, we would simply laugh at them, and continue on with what we are doing. If the budding Evil Overlord threatens to destroy the world, and everyone knows that he will follow through with his threats, then you know you darn well better give into his demands.
As we can plainly see, for the Overlord who always follows through, his job is already that much easier. The populace knows that he will do what he says. Destroy france, turn Iraq into glass, flood Australia, all amazing events, but you know they will happen if the Overlord says it is going to happen.
Always follow through with what you say you are going to do. Otherwise you will constantly have to do everything possible to maintain control. You will be forced to reach and struggle with everything, instead of having the people of the world giving you what you want.