Pitching a Campaign
In some senses, your one-timer will be written as a showcase game - intended to excite the players about the system and the setting and the campaign, and entice them to explore them further.
A crucial part of a showcase is identifying the key elements you want to convey. You need to figure out what your selling points are - the things you think will convince players the game is cool and fun and worthwhile - and also what you consider the core elements - pieces of the game that are central and important and constantly present, the ones that define the style and setting of the game.
Some examples of key elements could include:
- A D&D game might have strong focus on battle tactics, and a wide variety of surprising strategic scenarios. Or, it could be lighter on the mechanics and focus on high-adventure plots and finding lots of cool, powerful magic items. Clearly, if you intended your game to follow one focus, you wouldn't want the showcase to highlight the other instead.
- Primetime Adventures is a game that can run in all kinds of genres and styles, but what I like about it most is that it's great for running tense interpersonal drama. So whenever I showcase PTA, I always make sure to specifically choose a setting with lots of tightly-connected characters working at cross-purposes to each other, with potential for tense conflicts.
- If you had a steampunk setting, you'd want to get a few steampunk tropes in right at the start. If your campaign would be about battling the undead, you wouldn't want the showcase to be about training in Fighters' Academy with no undead skeletons anywhere around.
Once you know your crucial core elements, you can view those as constraints - those are what you'll want to build the adventure around.
A Good One-Timer
To work well in its own right, you want this to be a good one-timer, too. The key here is to make sure the game's fun right away, not relying on buildup to something that comes later.
You'll have to figure for yourself what the awesome, fun parts of your game are. Make sure everybody's got cool, enjoyable stuff to do; don't let any player fall by the wayside unless they're really not getting on well with the game.
Avoid, at all costs, constraining the players in accordance with any plots and plans you have about the campaign. Those considerations have no place in a one-timer - they don't have "well, this'll work out better in the long run" to balance them out.
Don't sweat minutiae of the setting or the system (unless you enjoy that). The game should be easy to get into - pre-written characters, with the GM running most of the mechanical side because the players don't know it yet. It's often helpful to provide a prose introduction to each pregen character's personality and abilities - because this is much easier to grasp then a full (and unfamiliar) character sheet.
Be sure to provide players with (at least) a default possible course of action. They don't know the setting yet, they don't know the system or everything they're capable of - so "You're a mob gang; somebody's just killed Jimmy the Kid - GO!" can really belly flop. "Someone's killed Jimmy - and if anybody can tell you who, it's Hysterectomy Sam" gives the players an initial direction until they're solid enough to have their own courses of action to pursue.
Tying Them Together - Structure
Since you want to sell the campaign AND have a good one-timer, you'll need a narrative structure that allows both. Here are some suggestions; obviously, some of them will be more appropriate then others, depending on the particular type of gameplay and campaign arc you're aiming for.
- Short Story: If your campaign arc is like a novel, or a series, you might be able to come up with a short-story scope as well - a complete story/adventure, just at a smaller, simpler, more modest scope. Not "journey across the sea of flame to discover the secret weakness of our terrible extraplanar overlord," but "rescue a prisoner the overlord's keeping in a prison made of echoing crystal." Same feel, narrow scope. Some game types are naturally broken up into small missions and sessions with clear beginnings and endings - e.g. Paranoia missions, police investigations, dungeon crawls. This might be a simple solution. Then afterwards, if they go for the campaign, you can break out the epic.
- Origin Story: One option is to use the first session as type of "origin story," in which the characters gain newfound powers. This is very typical of urban fantasy games like the various White Wolf titles or Unknown Armies - the first game has the characters discovering the fantastical world, and gaining new, exciting powers with which to take their place in it. Similarly, the idea of a group of heroes getting their start by discovering some great revelation or gaining magical powers works nicely as an intro.
- Adventurers Assemble: Much in the same vein, a "how the group is formed" game works well as a one-shot and naturally leads into a larger game. This could be something like "the king summons us all, declares us a team, and sends us out on our first mission," or "we're a bunch of slaves and we band together to escape." This type of structure demands a clear goal for the first game, and some process throughout the session which forges the group into, well, a group.
- Prologue: In books, you'll often have a short prologue - a story-like piece which focuses on some unknown characters, which is connected to the main story, but "off to one side," showing us some specific aspect of the world and plot. A one-shot can work really well as a prologue, and this way you can make the most of not being committed to regular characters yet. The one-shot could have characters facing off against the main villain (and being killed at the end!), or non-adventurer researchers making a cool discovery with dangerous side-effects (which the heroes will use, or deal with, in the main narrative). Come up with an "also happening in the game world..." idea; it'll be a lot less epic than your main plot, and it'll be an opportunity to showcase cool things that the main campaign might take a few weeks to build up too. The main thing to watch out for here is straying too far from the tone you intend for the campaign - e.g. if you're having an action-heavy robot-hunting campaign, then a story about the scientists who activated the first evil AI would be cool in an anthology or as a DVD bonus, but it wouldn't be a good introduction to the story all on its own. Most particularly, in a one-off you can kill off all your PCs, but don't do it in such a way that it sets a morbid, despairing tone unless that's what you want.
- In Media Res: In some games, you can get away with running a one-shot that explicitly takes place in the middle of the action, not at the beginning, not even on the side. Consider: if I wanted to demonstrate D&D to someone who knew nothing about it - would I want him playing a 1st level character clearing out rats for a farmer? Or would a better demonstration be a 6th level mage who's already got some fireballs to sling? Similarly, if your story's beginning is likely to be weak - you don't want to focus on the beginning. Throw them into the middle of the action, the intrigue, the excitement. You say "pretend a whole bunch of stuff has happened already; this is what the middle of the game would be like." You can't always get away with this - if you're relying on good understanding of the system and the setting mid-way, this just plain-out won't work. But if a session from halfway in would be kind of like watching a random midway episode of most TV shows - perfectly enjoyable, even if there are a few loose ends and unclear references - that might be a better path to exciting players than running a run-of-the-mill pilot from before the series hits its stride.
Hope this helps!