So, some of you may remember that a few years ago I posted some messy erotica. A few of you curious may have stopped reading at the end of that sentence to go and find said erotica. Those people will have found it no longer present. I've always thought that our fetish is underserved in the erotica department. I might have missed a big internet rock where there of thousands of pages of it lurking but I doubt it. This has always struck me as weird, because for many people finding a participant, setting, freetime and resources to actually get messy is a real issue. This doesn't include those people who just want others to be messy or want to mess up others. It seems odd that the above factors have contributed to the wealth of videos but not to a lot of fiction. Perhaps people don't have decent imaginations or something.
Admittedly writing good erotica is hard to do. The reason why mine no longer exists is because it was terrible in multiple ways. Firstly, It was written when I was 16 and uploaded a few years later with little (No) in the way editing. This contributed to an embarrassing number of spelling mistakes and bad grammar (typing one handed is hard). A secondary issue arose from them being written when I was 16, the female characters were completely 2D. Before I'd learnt anything about gender politics or knew women as more than erotic entities that wouldn't talk to me, turns out my fantasies could be quite sexist in nature. My Girlfriend pointed this out to me when I linked her to UMD for something. I woke up to a message asking me to explain why I'd never told her that I was a terrible human being for the stories. I explained how old they were and how much my attitudes had changed since then. She accepted that (And had a slight ego boost because the time frame for my enlightenment also corresponded with the time I'd known her.).
Anyway after this she asked me what my fantasies actually were. I want to try writing erotica again to attempt to convey some of these. The purpose of this post is to ask around to try and get some advice and pointers on how to do this well. Even if you've never written any, I think most people have gripes with things they've read.
(Also if anyone else wants to write this sort of thing consider this as a call to arms (Or is that pens? Or keyboards?)

Soooooo...

I would not recommend using fiction to communicate your fantasies to a significant other. No matter how good your (or anyone else's) fiction is, that just does not sound like a good idea. Other people's sexual tastes are often very hard to understand or appreciate, and you won't make that acclimation process any easier by throwing her into (basically) the deep end right away. Personally, I think that you should put some of that energy into really reflecting on your relationship to WAM and thinking of positive, confident ways to describe that relationship and how it relates to your relationship with your girlfriend. This last part is especially important, because many people have fantasies that they don't actually want to implement in real life, so it's important to think realistically and concretely about how your fantasies either would or would not mix with your relationship. All of this goes double if your relationship to WAM is predicated on anything like humiliating or overwhelming the "victim," as is true for many of us.

But.

That having been said, there is a place for WAM fiction, especially if it's well-written. Maybe you can get her to read some of your work if the two of you can successfully have a normal, straightforward conversation beforehand; maybe you'll end up writing some just because it's what you enjoy; whatever. And, as you say, part of producing good writing ("good" in both the artistic and political senses) is coming up with good (realistic, believable, three-dimensional) characters. So here's some advice about how to write good erotica.

-Know the limits of your scenario
WAM has a lot of dimensions: it can be fun and lighthearted, aggressive and challenging, purely sensual, and so on and so forth. Each of those approaches works on a different wavelength, and they don't always harmonize. Even if you yourself are attracted to many of these dimensions, in my opinion it's important to keep to just one or two when writing. The same goes for substances: don't necessarily feel like you have to hit all the checkmarks in every story.

-Find a believable psychological context for the action
In my experience, most WAM writers don't struggle to come up with a plot or setting. Instead, they have a hard time explaining their characters' motivations it any kind of plausible way. (Pro tip: "...and then she discovered that she kinda liked it" loses its luster after the first eighty or ninety times you read it.) This is especially difficult if you're starting from absolutely nothing - that is, if you're starting with a character who has no exposure to WAM and no sexual connection to any other character in the story. For some good examples of stories with good characters, see http://wamstoryarchive.freeforums.net/thread/566/cleaning and http://wamstoryarchive.freeforums.net/thread/1026/first-pie (If you decide that want to show your girlfriend the latter, you can mention that it was written by a woman. Should earn you some points.)

-Especially for new readers, work with the questionable gender politics, not against them
Yes, there are sometimes some questionable gender politics involved with some parts of WAM. It happens; let's not now turn this thread into a flame war. But those political undertones don't have to be presented naively (or, worse, exploitatively) in fiction. There are erotic ways of reversing or undermining those political issues, but you can only do that if you face up to them rather than pretending that they don't exist.

So there's some stuff to get you started. Seriously, though, start by just talking to her. Save the other stuff for the point where you both feel that you're comfortable enough with the reality of the situation to start exploring it as a fantasy in fiction.

I've written a few WAM stories (they're posted here), but I haven't asked my wife to read them (she knows what I'm into, though). Like larryniven said, it's probably better to just talk directly with your partner about what you want. The key word when it comes to sexual fantasy is fantasy: like roleplaying, fiction creates an imaginary space and what unfolds in it isn't necessarily "realistic," and that's OK. I also don't know that you should feel guilty about un-PC sentiments in your fiction if it accurately reflects what gets you off: see above about the difference between fantasy and reality.

However, I did find that attention to physical and psychological realities made my stories more satisfying (to me, at least), so it is worth taking the time to think about characters as people with desires and relationships that go beyond "Insert Tab A into Slot B" (or in the context of WAM, stories in which ten gallons of chocolate syrup is twice as sexy as five gallons). Maybe WAM itself is the field on which conflict plays out, or there's something else going on, but in my opinion even an erotic story needs a little drama.

At the risk of running long (too late!), there are really three things at issue: what is sexually satisfying to you, the state of your relationship with your girlfriend, and the creation of good fiction. It would be great if all three of those things lined up perfectly, but there's no cause to mourn that they probably won't. If your girlfriend feels comfortable reading your fiction, great, but don't expect her to take them as roadmaps.

Another writer chiming in here - my girlfriend found my stories did little for her, but that hasn't stopped her from being happy to indulge in some messy play with me.

That aside, on the writing fiction point - I love messy fiction purely because it does let you express your fantasies, and as others have said, the whole point to me of fantasy is that it doesn't have to be bounded by "realistic" limitations. While there are limits (I find stories of the type "what are you doing with that pie oh my god I can't believe you did that this feels good pie me more now i'm naked and want you to do me on a table covered in pies") in the space of about a page - stretching things somewhat; I also personally love writing about the messy action rather than the setup and the convincing a non-WAMmer to get involved (although I will recommend DustyBeard's stories as good at taking that approach). But from my perspective, if the thought of two girls (or more!) having a pie/slime fight and then getting off with each other turns you on (for example) - then why should you cater to anyone else's taste but your own?

I don't have any advice to offer on writing but I have used other people's stories to help introduce my partners to sploshing. When I first met Janet, she used some of the stories she had posted on http://www.literotica.com/ to show me her kinks. I showed this story to the chick I just recently started dating: http://www.savethebergamot.co.uk/mmm/wedding_day_detour.html

Given the right story, I think it can help show your partner your desires and fantasies

I've been looking for this story for a while now, as it's a personal favorite of mine from way back. Thanks so much for reposting it!