Why Willow/Xander ...
Someone once said that friends make the best lovers. If there's support for this statement on Buffy the Vampire Slayer, then it surely lies with Willow and her best friend, Alexander Harris.
No exact date has been given as to when Xander and Willow first met, but it dates at least as far back as their first day of Kindergarten together, when Xander comforted a distraught Willow after she broke the yellow crayon. What *is* known is that they spent practically their entire childhoods together, forming a close bond over that time that has lasted well into adulthood. Their only other friends up until tenth grade were Jesse and Amy Madison. And, thanks to decidedly poor family lives, the two were essentially alone in the world (outside of each other) for most of their childhood.
A major shift in their relationship occurred when Willow and Xander entered puberty, as Willow began to fall deeply in love with Xander, who remained oblivious to both her feelings, and his own. Despite this, Willow remained devoted to Xander, hoping that one day he'd notice that his best friend was quickly becoming a very beautiful (if horrifically shy) young woman.
Their lives changed again when they met Buffy Summers, the Vampire Slayer, and began helping her save the world. Jesse died and was turned into a vampire, which the two were able to deal with surprisingly well, but Xander quickly became infatuated with Buffy, who was in no way interested in a romance with Xander (and was, in fact, trying to push her new friends together, with little success). Romance nearly followed just prior to their junior year of high school, but a vampire and the return of Buffy from her summer vacation quickly ruined the chance for a Willow/Xander love affair.
As 1998 came along, Xander began a lusty relationship with Cordelia Chase, the onetime object of Willow and Xander's mutual disdain. A heartbroken Willow soon fell in love with Daniel "Oz" Osbourne, a guitarist in the local band Dingoes Ate My Baby. Both relationships developed with some level of normality (as far as can be had with Sunnydale's resident Queen Bitch and a werewolf) until Willow was nearly killed while attempting to restore the curse on Buffy's boyfriend, the vampire Angel. As Willow lay in a coma, Xander professed his love for Willow, only to have her call out for her boyfriend.
The incident was soon ignored by Xander, and their friendship continued until the two decided to try on clothes for the Homecoming dance in each other's presence. Willow and Xander kissed, and began sneaking behind Cordelia and Oz's backs until they were eventually caught. The incident nearly destroyed their friendship, and managed to completely derail Cordy and Xander's relationship-in this world.
Cordelia's need for vengeance attracted the attention of the Vengeance Demon Anyanka, a patron demon of sorts for scorned women. Cordy's wish for Buffy to have never come to Sunnydale resulted in a darker world where the Master was able to exit the Hellmouth and walk the Earth again freely. Ironically, while Willow and Xander did die in the resulting upheaval, they were reborn as vampires-who as much in love with each other as they were evil. This alternate world was eventually destroyed, and things returned to their normal course of events, although Anyanka was now the human Anya Emerson.
Willow and Xander's friendship continued to crumble at this point, as Xander lost his virginity to the rogue Slayer, Faith, again crushing Willow when she eventually learned of the tryst. Willow deepened her affair with Oz, eventually consummating the affair the night before their graduation from high school, and Xander drifted off by himself, eventually leaving Sunnydale for a summer.
The strained friendship between Willow and Xander further crumbled, as Xander quickly entered a relationship with Anya upon his return to Sunnydale, and Willow buried herself in her collegiate studies after her breakup with Oz. At this point, Willow fell in love with another woman, Tara, and Xander and Willow's friendship soon bottomed out (as did both of their friendships with Buffy and Giles), thanks in part to the interference of Spike.
However, Spike's gambit (done at the behest of ADAM, the Initiative's Frankenstein-like creation) failed, and the four core Scooby Gang members quickly rebuilt their friendships, Willow and Xander included. Things became strained between the two again as Xander's wedding plans with Anya progressed to their tragic conclusion, and Willow became heavily addicted to magic. All was forgiven, though, once Xander told a vengeful and grieving Willow that he loved her just as she was about to destroy the world.
As a fanfic writer, there's certainly a lot to work with in regards to Willow and Xander, both in the past, and in the future. They have, and continue to, love each other very much. It's not a stretch of the imagination by any measure to see their freindship growing into a full blown romantic affair. However, there are a number of things one must keep in mind. They include:
1) *Never* discount Willow and Xander's previous lovers.
Many fics (especially those written when the "Tara is really a demon" rumor was still alive and well) tend to dismiss Willow and Xander's current love interest simply to ensure that the two are conveniently single at the start of the tale at hand. Although there are exceptions, taking the time to show the breakup in question is always a good idea. Another recommended step for writers dealing with the current post-Tara state of affairs is to not dismiss Tara as an "experiment" (more on this later).
2) Silly/funny/romantic/etc. memories from Willow and Xander's past are your friend (and also your worst enemy).
Willow and Xander have goads of history. Use it. But use it *wisely*. One memory per fic is a good standard. Just remember this: don't overload the "major days" of their friendship (i.e., the first day of Kindergarten and their 8th grade cotillion). Restraint is always the better part of valor.
3) Buffy=The Voice of Reason.
Buffy loves both Willow and Xander. As such, she wants to see them both happy. If your fic takes place in Seasons 1 or 2, Buffy will be actively pushing for the two to get together. In later seasons, she simply wants them to be happy, whoever they go out with. Plus, given her lovers (Angel, Owen, Parker, Riley, Spike, etc.), she can be the source of some really great advice.
4) Giles=Dad, and Joyce=Mom.
Giles and Joyce are the only parents that all of the cast, Xander and Willow included, have ever had. If one (or both) are present, and you need a more authoritarian voice, go with them. Strange as it might sound, Moms and Dads have good advice, too. On the other hand, you can freak them out even more than Buffy by having Willow and Xander going at it in an excessive manner. :)
5) Willow and Xander need to grow up.
It's not so much the case coming into Season 7, but if you want some W/X 'shippage, you need to mature one or both of the two. Some of the best Willow and Xander fanfics take place in the future, believe it or not. Many of the other good ones dispense with the BS and throw difficulty and loss right into their very not ready faces. But the key factor is adversity. What doesn't kill us only makes us stronger, and all that.
6) Learn from the "masters".
There are a lot of really, really, *really* good writers out there in W/X land that are worth reading, including (but certainly not limited to) Sarah Jones, Christi Mallasch, Nikki, Palaskar, Rae, Pete Milan, Michael Bujanowski, Loki, and Pete Meilinger. Educate yourself and know where people have taken the 'ship, and, more importantly, where they *haven't*.
After you've followed these suggestions (or laughed at them because you think they're stupid), one issue is left: Willow's sexuality. As of this writing, Joss Whedon and Marti Noxon have decided against further exploring Willow's sexuality beyond her being gay, largely in the wake of the hell they caught from Willow and Tara 'shippers after Tara's death. Thing is, you're reading this in the hopes of getting advice writing Willow/Xander fanfic. So, either Joss has proved to be a big fat liar (again), and Willow's sexuality is being questioned (meaning this essay needs a bit of updating), or Willow's gay and only gay in canon.
Without going off on my thoughts of the decision itself, I'll say this much: the evidence implies succinctly that Willow is bisexual. So, in the crudest possible way to put it, she gets off on guys and their guy parts just as much as she gets off on girls and their girl parts. :D
At the core of this argument lies our initial hint that Willow might be gay: Vampire Willow. When we first saw her in "The Wish", the character was very much involved with the Vampire Xander, with both getting off on torturing Angel (which seems like VampXander's idea-I wonder why..... ;)). In "Dopplegangland", VampWillow hits on everyone-including her human counterpart! Gender is certainly not a barrier for her, and as we learn from Angel, a vampire's personality is a reflection of the personality of the human that once inhabited that body.
Moving forward, we have Anya's accusation to Willow in "Triangle" that Willow might eventually steal Xander away from her, which was followed up by the infamously nervous line, "Hello, gay now!" Similarly, Oz pointed out to Willow in Season 3's "Amends" that things may *never* be over between her and Xander. Apparently they give off a vibe or something, because Oz and Anya have connections to supernatural forces, to say nothing of the fact that they're pretty darn smart, especially when it comes to people.
Willow has made other ambiguous comments-getting angry that Xander is just figuring out that "smart chicks are hot" in "The Gift", taunting Xander with the dreaded ".....for the rest of your life" in Season 6 in regards to Anya's more.....annoying habits, and the "it's good I'm gay now" comment during their tender moment in "Hell's Bells" implies that there's still some latent attraction for guys (and Xander in particular).
However, Willow's sexuality is not a light subject. This is someone's *sexuality* here, not their favorite color. Who Willow finds sexually attractive is at the core of her character, and therefore dictates the success of your fic. I'm not saying you should muddy up a fluffy fic with the sexuality question, but any serious fic taking place between seasons 4 and the present demands that you at least address the concept.
My last word of advice regarding Willow and Xander fanfic is this: have fun. Xander is easily the silliest character in the Jossverse when he wants to be, but he's also one of the sweetest, loyal, and troubled characters as well. His relationship with Willow is unique, and perhaps one of the few truly deserving of the moniker of "soul mates". This pairing is for hopeless romantics, but there's definitely nothing wrong with that. :)