I am preparing for a Star Wars game right now (well, right now, I'm procrastinating on preparing for my Star Wars game by writing a message board post about how I'm preparing for my Star Wars game, but you get the idea). Here's some of the stuff that I've done so far.
First, I ask myself what I want out of the campaign, the session, whatever.
The campaign takes place during the clone wars. The characters are a pair of itinerant Jedi, some Clone Troopers, and some technical experts that have offered their services to the galactic republic. My plan is to get them involved in various scenarios tangential to the meta story, and see how they effect things. It seems likely that they will bash into the canonical story and jack it all up, so that is what I'm (more or less) planning for.
For my initial inspiration, I went on Wookiepedia and bounced around on random pages until I found something that interests me, and then I combined it with some stuff that is related to the main storyline of the Star Wars prequel trilogy. Doing this, I found a system called Seoul (the same name as the capital city of South Korea, where I lived briefly). So I looked into that. There, I found a story where Han Solo fought some Imperials doing weapon research on some mysterious force crystals from a ruined civilization. That sounds promising, but will require some adaptation for the current scenario.
Also, I know that I would like it if the player characters eventually thwarted the assassination of (former) supreme chancellor Finis Valorum (played by Terrence Stamp in the movies), so I want to give them a taste of some corruption in the government and a hint of the massive conspiracies that are taking place right under the jedi's noses. So some of the characters involved in that assassination are going to pop up in a few of the initial adventures.
The first session (which I'm planning right now) is to be a murder mystery (sort of). The characters have returned to Coruscant from a previous mission (I am actually taking over the campaign from someone else) for a debriefing by the Jedi high council and whatever else may be going on. While there, they will be approached by another jedi, asking them for a favor. (I'm using Jedi master Plo Koon, but it doesn't really matter who it is). This is the plot hook.
One of the Jedi master's friends, an Antarian Ranger, has been murdered, and the Keldoran Jedi, being busy with the war effort, has asked these itinerant jedi to investigate on his behalf. All that is known so far is that the Antarian Ranger's starship exited hyperspace in the Coruscant system with its owner deceased. It has been taken in to a Republic Office of Criminal Investigations impound lot and the body delivered to a medical examiner (or whatever the Star Wars equivalent of that is). So the players have two avenues of investigation to start with.
The first stages of the adventure will be mostly social, in nature, with the characters going to talk to different folks and finding out what they know. I'll use this time to add a little flavor, and make notes if the players make a particular connection with any of the npcs that are introduced, for later use.
Eventually, their investigation will lead them to the Seoul system, where they will discover the larger plot going on. Some unfamiliar clones (based on a different prime than Jango Fett), led by a couple of Anzati assassins will be going about some nefarious business, attempting to turn the ruins of this ancient civilization into some sort of superweapon (star wars is all about the super weapons, it seems like all anyone in that galaxy wants to do is blow up some planets).
The middle parts of the adventure will involve the players negotiating the ruins and possibly fighting a bunch of clones and several Anzati assassins, as well as a dark jedi. Hopefully, they will figure out some way to stop these guys from building whatever super weapon is being cooked up (in the original version of this, Han Solo accidentally blew up the planet, but I'll roll with whatever these guys do). Also, they should encounter the person who actually did the murder, so that they can consider their objective completed.
The end parts of the adventure should involve wrapping up and reporting what they discovered to master Plo Koon, and then something should go horribly wrong, setting the stage for the next adventure.
Most of the proper nouns I'm sprinkling into here are things that I found while clicking around on the wookiepedia site, but if I was using a homebrew setting, I would be making these sorts of things up myself. With the overall structure of the adventure sorted, all I've got to do is whip up some NPCs (impound guy, medical examiner, information broker, alternate clone trooper, anzati assassins, dark jedi, maybe some hostile native life forms), come up with a map that looks like it could be the weird power station for an ancient civilization, and develop a few contingencies in case things go differently than expected.
Now, because my group meets for a relatively short period and because my players screw around a lot, this scenario will probably last for about three sessions, so I don't need to be in too much of a hurry. But, as I do not have to work on Christmas eve, I will probably do the rest of the necessary prep (which is mostly all administrative stuff, rather than anything creative) tomorrow.