There is a balance you can meet between pre-gens and player created characters. How much work it takes on your end and how much time it steals from the session depends on how high level the characters are for the adventure.
1) Create 1.5 times as many characters for the number of players you have, covering all the bases and niches, plus any 'neat' classes you or your players happen to like. Give them stats on the point buy, give them (base+int mod+variable) skills at max ranks. If they are higher level, give them appropriate feat chains up to half their available feats.
2) Create equipment packages that include the stuff they'll need, but with built in wiggle room (say "+2 magic weapon" instead of "+1 flaming bow") and don't forget potions and or scrolls. You'll need to create at least one package per above character, but if you have time, more wouldn't hurt.
3) For wizards (or any other class that needs to learn spells) create a spellbook and allow for one or two additional spells of player choice per spell level.
4) Stat up any animal companions, mounts etc... as necessary.
When the players arrive, give them a half hour or so to name their characters, choose remaining feats and adjust skill point allocation, and allow them to personalize equipment lists and/or spell lists. You can do the exposition part while they are working, so that once they are done, ou can jump right in.
Because the players get some choices and have a hand in their characters, if it does turn into a campaign, they feel more of a connection to their "creations" than they would characters thrust upon them. One of the key components of 3.x is player empowerment, and while it doesn't matter much in a one-shot, it is very important in a campaign.
On the adventure side, make sure that you are prepared. Nothing makes the (recent) addage of "four hours of work for 20 minutes of fun" truer than a DM having to scramble to keep up with the players. Have all your stat blocks, spells, etc... on index cards or in a legible list. keep your minis/token/whatever close at hand. You might even be able to predraw the battlemat (especially if you have ACAD and a large format printer
). As others have stated, a strong hook that makes the players want to be railroaded into the adventure is a must.
Don't go overboard in designing the adventure. Once you have all your encounters figurered out, come up with ways to bypass them and still get the whole experience of the adventure. You don't want to stop in the middle, so if early encounters take longer than expected, you want to be able to shuttle them to the boss fight/climax easily. If you want a combat heavy game, use lots of low powered mook type enemies until the final battle. Mook fights are fun -- players get to kick butt -- and are generally fast. Save the long, uncertain battle for the end.
And one last piece of advice: depending on the timing of the one-shot, have everyone eat before, or agree to get dinner afterwards. Even if you try to keep gaming while you are eating, getting pizza or throwing burgers on the grill can kill an hour or more of game time when all is said and done.
You haven't met the people I play with!
