A well-organized plot enhances not only novels but also many an RPG adventure. It's not easy to stick to a rigorous plot while maintaining the illusion of free will however. Especially in event-based adventures it can really seem like the game master is trying to force certain things to happen despite the player's actions. This happens very often in games I've seen. Here's some tips for making the skeleton of a campaign and filling in the rest collaboratively. Overall arcs, planning open-ended conflicts, dealing with tangents, foreshadowing and finalization will be covered in that order.
Our first step is to get a picture of the adventure in it's entirety. It's easiest to start a puzzle with a picture of the finished product, correct? Event-based adventures are the same way. Draft up how you would like the adventure to begin and then draft how you would like to see it end. Once you have those then bridge some of the gap. I say "some" because we're not writing a novel here – we're working on only one part of an RPG story. Let's use an adventure starting with the princess being kidnapped and ending with the ogre chieftain being defeated and her being rescued. That might be filled in with things like finding where the ogres live and learning about the ogre chieftains weakness. Maybe they have goblin slaves? In that case add 'free the goblins so they won't be an issue when the players storm their lair.' Don't flesh out every detail. Paint this in broad strokes just to give yourself a general picture of what you'd like to see in the adventure. The more narrow and concrete your design scheme then the less interactive the adventure will be for the players.
Each of those bridges you just outlined represent plot arcs. However, there's a slight misnomer in the phrase "plot arc." While it's true that we want to add story and intrigue to your adventure it's also true that we don't want to force or presume your player's actions. We need to remember to plan unresolved conflicts and not pathways. Planning conflicts takes an extra step but it's well worth it. As the adventure begins to appear less unchangeable, the players will be more engaged in working to change it. The extra step is simply to make two or three sequences each stage in the event-based adventure might go. For example, instead of 'Abbadon steals the crown jewels and flees,' go with 'Abaddon tries to steal the crown jewels.' The players might stop him. It'll be difficult but not impossible. If they do stop him then he might have an escape plan to try again or those jewels might have been a decoy. His real plan could be taking the players far from where his henchmen are stealing something else. Planning conflicts also leaves you a great deal more flexibility with the course of your adventure. Your players will have more influence in the game world and you'll have to try and force less to fit. Ideally there should be nothing forced to fit and you can just shape the details of the adventure around their actions.
In video games tangents can be more fun and even more rewarding than the main quest. Side quests fill in the rest of the world and sometimes more of the main story. I've got to say though that if one more newly released video game insists I run errands for over half the game I'm going to send letters in protest. Role-playing games have an even higher constraint for this: Gaming with friends involves time commitment and mutual devotion. Unless all of your players live for those tangents keep them to a reasonable ratio. I can't stress that enough. Side quests are great but keep them pertinent and make sure your players will never lose sight of the main quest once they get to it. How terrible would it be to miss saving the world because your party is busy tending the town farm? Examine your plot arc list and keep the main quest shining through with side quests aiding it as directly as possible. Unless your players teamed up to run a delivery service, that is.
Foreshadowing is the part of adventure writing that I enjoy the most. Since that's in the open I've made some mistakes here and there you can learn from. Foreshadowing is a keen art that if employed properly can really amaze people. First thing's first: Less is more. If anything major in the game is being hinted at all the time then your players might catch on a bit too quick. But if they're looking for foreshadowing and find little then they will have less to work with. Pick the most major things in your game and then find somewhere earlier in the plot to put them. Do they have to fight and kill an ancient dragon? Maybe a few steps before that arc they will be able to see it flying on the horizon. Perhaps rumors exist of it before then. Especially for major battles foreshadowing can serve two purposes: warning of impending danger and potentially alerting to a weakness or two. Just try to keep the mysterious cloaked figures they meet at the very beginning of a game to a minimum.
It's so nice to have an adventure skeleton laid out before you. In-game and preparation value both are worth more than the time invested painting the bigger picture. Take a six-step adventure as an example. Put it on display and organize the foreshadowing, side quests and plot arcs. Now you don't have to think of anything beyond what's happening at this and the next arc. Thus you can focus on the session at hand. Maybe plot arc two (we'll say free enslaved goblins) and five (learn of ogre chieftain's weakness) are foreshadowed at the beginning of the game and plot arc one (princess kidnapped) is coming up soon. Instead of thinking of all six plot arcs and how to foreshadow and portion side quests to each (or any) all you need to concern yourself with is foreshadowing two and five and setting the players up for plot arc one. Foreshadow freeing enslaved goblins by hearing of their revolt being crushed, reveal that the ogre chieftain has a weakness by having him silence a minion just before they mention it and bring them toward the site the princess will be potentially kidnapped from. The crown jewels may as well not exist in this session. Same with the location of the ogre lair and the name of whatever powerful artifact the ogre chieftain might have. Nothing else matters during this game session and that makes it so much easier to improvise. Of course all those other things matter when they come up but it's both easier and more fun when a game master has to think only of the current game session. Organizing an adventure's plot hooks helps you organize yourself at the table and your players will see that. Happy weaving!