From what Ive seen it tends to go one of three ways:
Made up words: Frak, Frell, Dren, Gorram, etc. These need to be similar enough to common curse words that the reader gets what you are hinting at, without pissing off any censors/editors involved. Not bad but it is rather obvious by nature, which is why you typically only see it with censorship pressures.
Cultural: Great Furies, Merlin's Beard, Empty Night, By the First Egg, In the Name of Zeus's Butthole, By the Blood Tears of Lisen, etc. This type of swearing makes sense only within the context of the story itself. Its predicated on the idea that swear by things important to the culture, usually from whatever passed for myth or legend.
Local/idiomatic: Crow-begotten, thrice-damned, two faced son of a <insert insulting comparison here>, etc. These are your general curses that could basically work in the real world too, if something made them popular enough. Crow-begotten was common in Alera because of the cultural awareness of crows and war, but the reader doesn't really need to know anything about the story or setting to get the point.
Not sure that really helps, but its the two cents I have on me right now