On the sci-fi channel's FarScape show they made up some new words, such as "What the frell...." If you take a couple of letters from a real swear word and add some different letters after it (or pick a different word that starts with those letters) you can have the feeling of cussing, without actually doing it. "Oh shards" "Fudsucker" "mudsucker" "Oh frack" "What the flick?"