Though some people frown upon it, swearing is part of most languages. I can only speak English, but I know that when speaking English and in the U.S., the act of swearing heavily depends on the surrounding environment and people. It's considered taboo in many situations, and that's when fake swear words come in handy. The thing I really like about seeing them used in novels or on television is that it makes me think about language. When I'm saying “frak” in a public place, I'm really saying something else. It could be any substitute word though. It's the feeling behind it. It's very arbitrary.
I could continue an analysis of language, but instead I'll just give you a list of my favorite fictional swear words and phrases.
Film & TV
Frak - I attended a BSG panel at San Diego Comic-Con a couple of years ago. Kevin Smith moderated the panel, and I specifically remember him giving Ron Moore “ups” for using it for an obvious substitute and having it be acceptable.
Example: Mother-frakking toasters!
Frell - Substitute for the infamous f-word.
Example: I want this miracle of life the frell out of me
Example: Did the primary buffer panel just fall off my gorram ship?
Books & Comics
Wheel of Time
Blood and ashes
Mother's milk in a cup
Sheepswallop and bloody buttered onions
These are all usually used as stand alone phrases.
The Sword of Truth
Example: Bags, and Double Bags!
Star Wars: Legacy
Example: Kark it!
Phoenix Guard or Vlad Taltos books
Cracks in the Orb!
Example: Cracks in the Orb, haven't the Dragon wizards proved it?
Get out there and don't offend people with the new words you've learned!
Feel free to share more fake cuss words in the comments. I know there are lots more out there.