Pick up some books any genre and you can find a formula. Patterns that authors follow to tell a story. In romances with vampires or werewolves, one formula I've noticed usually goes like this: Human meets supernatural creature who hides truth from human. Love ensues. Human finds out truth and feels betrayed. Human runs off and gets caught by bad guy. While in grip of bad guy realizes supernatural creature isn't evil and does love creature. Supernatural creature comes to rescue. They declare love and live happily ever after.
Recently I've figured out a formula I like to use. It goes like this: Boy goes through life like normal, but feels something is off. Boy learns he is not human and a new reality is revealed. Bad guy enters story and causes chaos. I've done it in Out of Secrets when JJ meets Cage and learns vampires exist. I do it in Snapshots, but I'm not going to tell you how because that's giving away part of the ending. In Puck's story I did it when he learns he's fae. Even the current WIP I have Hadrian learning that the truth he's been told is a lie.
Now formulas aren't bad, but an author has to be careful. There needs to be enough difference in the details so the reader doesn't feel like he or she is reading a rehashed story. Let's take my two newest characters, Puck and Hadrian. How are they different? Well, Puck's pretty much a normal guy. He has friends, he's on the swim team, both his parents are alive and happy, and he's dated a few different girls over the years. Hadrian, on the other hand, is an orphan and has been isolated. He's part vampire, part human and that has caused the people around him to shun him, the shunning encouraged by his teachers. So when both their worlds expand and things revealed, they both feel a sense of relief, but their reactions are different.
Another way to keep the formula fresh is with bad guys and minor plot lines. Give the villains different motivations for their evil deed. Have something distract the main character and before he can save the day, this little loose end needs to be tied up.
Writers all fall prey to formulas and you can find formulas in any genre. But they are just a part of story telling. If the story is compelling, gripping me and refusing to release me until I get to the end, I can honestly say I don't care if it follows a formula. I can only hope that my stories will be just as gripping, that even if a reader says, "Yeah, she followed this formula." that he or she will still have enjoyed the ride, that it was different enough to not matter.