Cliches are like insects hiding in your shoes. Sometimes they're harmless, sometimes even cute (like a ladybug, aaw) but sometimes...sometimes your entire foot will go gangrenous if it--okay, I can't carry this analogy all the way. But you get the point.
Playing with cliches can be fun, as Anton said. The very title Personal Demons plays on the cliche in several ways and alludes to more. We have the actual personal demons, the Yezer Ha-Ra. We have the demon bodyguards assigned to Megan, who become in essence her personal demons. We have the demon love interest of the story, who gets very personal. We even have Megan confronting the literal (and figurative) demons of her past.
So yes, cliches can be helpful and fun. It's fun to twist them and turn them.
But it's also deadly to use them too much. It alienates and bores readers. It lacks snap. It makes them feel like they're reading a book they already read.
In that book, I have a scene in the autumn woods, with bare trees. I originally had the trees "like sentinels along the road", before it was pointed out to me that that is such a cliche. Yes, it's evocative and it works, but can't we think of something better? (I had to go back and check: Now the trees stand in twisted columns along the sides of the bumpy road.) Trees standing like sentinels make me think of the Ents in Lord of the Rings. I certainly don't want someone reading my book and thinking, "Hey, maybe I should go read that now." (Um, note to self: Perhaps edit woods scene to remove characters discussing Lord of the Rings. Yes, they really do.)
Cliches are extremely useful in dialogue. When characters speak in cliches it tells us so much about them. F'rinstance, if I bring a female character in and, with no other description, have her utter the line, "I never heard of such a thing in all my born days," what do we know about her instantly? We know she's probably at least middle aged. Southern. Might wear a hat. Is angry/indignant about something or disgusted with the behavior of someone, so chances are she's a busybody. The mere fact that she's speaking in a cliche tells us she's a traditional type. She may not be stupid or unimaginative at all, but she has a set way of looking at things.
Whereas if she'd simply said, "I can't believe so-and-so is doing whatever", we wouldn't get those clues. We'd need description of her to tell us how she said things, how she walked into the room, what she was wearing. Description is fine, but we'd rather not waste paragraphs on her tidy iron-gray curls, calf-length flower dress, or squeaky, sensible shoes and matching bag if we don't have to. We can slip those in later if we need them, because that one little cliche has told our readers what sort of person she is.
Similarly, a character who says things like, "Turn that frown upside down!" or "It can't be that bad!" or even, (shudder) "Smile--God loves you!" is automatically an insufferable and irritating do-gooder of the type who deserves a messy death. What I see when I hear those phrases are big, shiny teeth, strong from uncountable glasses of milk. I see homemade knit caps and prissy tucked-in shirts (which the character calls "blouses", and which probably have Peter Pan collars.) I see shelves covered with knick-knacks instead of books and weekends spent scrubbing floors by hand.
What do you see? What's your favorite example of cliche used to effect?