A good measure of how popular a series is is the amount of fanfiction it generates. Just look at the Harry Potter and Twilight fandom. Heck, even Fifty Shades, which started out as fanfiction, has fanfiction written on it! It means people are interested in the novel and feel close to the characters, which is just what you want.
Since most fanfiction writers are teenage schoolgirls, you must know how to pave the path to ensure countless fanfics are written of your characters. One of the most popular sort of fanfics is to ship off people in relationships which may be hinted at or not even mentioned in the book. For example, those who belong to the Jacob-Bella ship. As well as Edward-Bella (because some of the books were written at a time Bella was fancying Jacob and Edward had left her). I suggest you ensure one of your male characters fancy the heroine. Make him attractive so fans will go for him, but don't make the heroine too attracted to him. That will make all the shippers go nuts and resolve this problem namely by writing it out themselves. Don't make him too minor a role, or else everyone will forget about him. If there is uncertainty as to whether the heroine fancies him or the hero, even better. I myself have long harboured a wish that Neville Longbottom married Luna Lovegood, but they were ill-paired. According to the rules of fiction they ought to have been paired off. Also, some people have wished to pair of Leah from Twilight with Jacob I think.
Fanfic writers are also very fond of writing slash, whereby two characters do gay stuff with each other. Two close guy friends (or girl friends), especially with one of the friends harbouring an unrequited love for the other, will ensure this sort of fanfic gets written. If the gay character is likeable the shippers will want him to get his love, even though the other guy is straight. This is why both admirers of the heroine must be major roles - so the shippers will make them do gay things. For more interesting results, the Mother Figure must be unusually attached to the Heroine, which will result in lesbian fics.
Since steamy sex scenes are popular in fanfics, you must take the opportunity to get more fanfics by censoring all the sex scenes in the novel. Better still, put in none at all. This will understandably frustrate the more lurid-minded readers, who will then write an even more lurid fanfic. Unsatisfied passion sells, as ascertained by Twilight and its fanfics.
Fanfics are notorious for altering a character beyond recognition. See Draco in Leather Pants or Ron the Death Eater. Make one character deliberately boring, so he'll be re-written as insanely evil. For the unbelievably evil character, make him hot so people will be rooting for him.
But not all fanfics are about relationships. Some fanfics are used to propagate the author's ideologies or emphasise modern social issues. To get an idea, read My Immortal, Twila: the Girl who was in love with a Vampire, and Forbidden Fruit: The Temptation of Edward Cullen. Harry Potter arguably has too many perfect Mary Sue characters who achieve so much (and they are the heroes) and the Slytherin characters are often morons. Outraged readers will re-write Gryffindor as evil and Slytherin as good, causing even more controversy. I've often thought what self-centred jerks the Gryffindors were, and those who are quite nice people are written to be too perfect. Basically nerds like Neville Longbottom ought to be in Ravenclaw or Hufflepuff but get put in the elite class, Gryffindor. There seems to be no outstanding intellectual from Ravenclaw, which are famed for their intellect. All the interesting clever people are from Gryffindor. Yes, Gryffindor is for the perfect, popular way to success. It's for outgoing go-getters with brains and charm. And some turn out to be jerks. Slytherin is supposed to be Nazi Hitler, but I could argue the case for Gryffindor as well. What about Ravenclaw and Hufflepuff? All ignored, despised nerds and geeks. If your novel is riddled with silly Mary Sues, you will get many haters. This is actually a good thing because it means publicity. Your supporters will defend you even more strongly.
As I was saying, Twila and Forbidden Fruit deal with rape and perversion and depressed teenagers, unlike the original Twilight. Doubtlessly the authors decided the original wasn't good enough at tackling these issues, and wished to get their revenge on Stephanie Meyer. Mind you, the paedophilic bestiality touch was pretty good, involving Jacob and Renesmee. Outraged moralists exclaim, and then you must congratulate SMeyer for putting in these social issues. Also lots of people who hate Renesmee are now shipping Jacob with Leah.