Well, Ive never run such an adventure myself either, at least nothing short-term, but I have played in one. If you're interested in actually reading it, it's the Savannah Knights Storyhour in the Storyhour forum:
http://www.enworld.org/messageboards/showthread.php?s=&threadid=87
But anyway, let me say at first that it's hard to do a satisfying mystery that is only one session, since the party needs time to stew over the facts and eventually come to a conclusion. If you go for more than about 4 sessions, though, the mystery becomes more of a diabolical plot. If a mystery can absorb the PCs for so long, it's probably more the basis of a campaign, rather than one chapter along the way.
My suggestion for designing a mystery is first start off with life as usual, then toss in something slightly amiss. Have encounters that don't appear to be directly based in the mystery, but that actually do have some type of link, maybe by introducing a possible suspect, giving a clue, or again cueing the party in that something is wrong.
Then, after a few such encounters, if the PCs haven't taken the hint yet, have them get directly embroiled in the mystery. Maybe the PCs go to confront the Ogre Magi, and are almost hurt by the defending forces protecting the town, or perhaps they find a child crying in the woods, who hysterically refuses to go home to her parents.
The key thing is to make sure there are a lot of possible suspects. Maybe they hear rumors that a ghost has been prowling the city, attacking indiscriminantly, and it has been attacking both villagers and the Ogres. If they know that an Orc tribe is looking to raid the town, and hear that a powerful sorcerer left his position at a local magic school a few weeks ago, the PCs might think the Orcs hired the sorcerer to weaken the city's defenses. They might even hear that the Ogres were allies of the city, but a power-hungry noble who wants the town's land for his own made a pact with a demon to scare off the villagers.
Then of course include a few less obvious rumors to be misleading red herrings--the well was dry for three days last week (in truth, some kids were mischevious and just tied a knot in the rope to shorten it), baker Turlus's business has been doing a lot better than it should (coincidence), a group of three children were buried together last week (the villagers claim it was an accident and they all drowned together, but in truth the evil god demanded one child be killed for each Ogre to be destroyed; the third boy did drown, making a convincing lie).
Have a lot of small rumors, but don't make big deals out of them. A good mystery sleuth will know that one of the less obvious answers will be the right one, so you need to have a fair share of these. If possible, have the clue be the right one, but point in the wrong direction. Maybe they spot a group of hooded people leaving the church where the rituals are performed, but lose track of them; when they later spot a similarly-hooded rider come into town, claiming to be a messenger from the local feudal lord, they'll assume he's involved. In truth, the hooded figures are the key to answering the mystery, but the PCs will never even guess they're actually villagers.
The final piece of advice is to be flexible. The players might for some reason be hell-bent on the idea that the Ogre Magi are trying to frame the mysterious events in the town on the above-mentioned noble, so that the noble (who is actually a loyal defender of the town) will be removed from power; this would make conquering the town easier. When the game reaches a break (maybe at the end of a session, or a pizza break), sit back and ponder how you could tie their wild guess back into the real mystery. You might decide that, in truth, the noble
is involved in the pact with the evil god, and the Ogres want to get rid of him because then the pact would be broken. This way, the PCs are almost right, but far enough away that the ending would be a twist. When the rumors of the noble having a pact with evil begin to spread around town, it would surprise the heck out of the party if the townsfolk casually go, "Yeah, tell me something I didn't know."
If you're flexible, and have a good sense of drama, you should be able to come up with a satisfying climax. In fact, come up with a few possible climaxes ahead of time, in case things stray a little bit. Never let the party be entirely right, but try to make them feel as if they were at least on the right track.