Plot Generation Tips
Choose the Style that Works Best for You
Every gamemaster has their own methods for creating an adventure. Some GMs create a complex story, with a defined goal and predetermined challenges the players will face. Some GMs start with a more relaxed format, simply knowing the end goal and a few different paths the players follow to reach it. They can then fill in the details of each path as the players choose a particular course. Other GMs prefer to let the players lead the story, simply leaving breadcrumbs of different storylines and seeing what the players decide to pursue.
Whatever style works best for you, it is important to give players some sort of path to follow: a goal to achieve, problem to solve, or oddity to investigate. The plotline can be simple or complex, with a single short term goal or multiple long-term goals. An adventure can be as short as a single session, last a few weeks, or go on for months. Typically the type of goals the PCs have will determine the length and breadth of the adventure, but you can also have strings of short goals that end up leading the PCs on a long lasting adventure.
Sandbox Storytelling
Some Gamemasters prefer to let the players lead the story, and instead of having a definite goal in mind at the start of an adventure, have several potential goals or just the starting points for story threads when the adventure begins. This is sometimes referred to as Sandbox Storytelling, as the player character's are simply put into the story world (the "sandbox") and left to discover their own goals based on the story threads that you provide. While this may seem different than the goal oriented storytelling described above, at its core it's really just a different version of the same thing. In this case, a GM simply creates a series of potential kick-off points for an adventure to see what sparks the player's interest. Once players have dedicated themselves to one (or more) specific story threads, then the GM will flesh out that story and form concrete goals along each path.
Character Motivation
Ultimately, the key is to create a plotline that is interesting and fun to the players (and their characters). When building your adventure, it helps to have some idea of each character's motivation for adventuring. If a character is motivated by honor and glory, the plotline should provide them some means of achieving that. If the character's motive is riches, the plotline should provide some hint of riches to be gained from it (or at least a means to riches). By knowing the character's motivation, you can then design your plotlines with these motivations in mind.
Alternatively, if you already have a definitive plotline planned out, you can give the player's a prompt that will help tailor their expectations and give you insight into their character. For example, "You are a member of a secret organization that aims to overthrow the evil tyrant. How did you come to be associated with this group?" In this case, you can continue with your storyline about "overcoming the evil tyrant," but can work to make it more personal for each PC.
It also helps to ask each of your players to create a Character Thesis, which is a word or short phrase that represents their character's strongest and deepest motivation. Ultimately, everything they do is first fueled by this notion. When you are creating the story, ask yourself if the expected course the players should take aligns with each of the Thesis' or not. If not, you may want to make adjustments to your plot to make sure that your character's actions are in line with your expectations.
For more on how knowing your Player Character's motivations can help you in starting your players down the adventure path, see Adventure Hooks.
Allow for Changes
PCs don't always follow the path as intended, sometimes they will surprise you with new and different ways to accomplish the goal that you laid out for them. As a GM it is important to be receptive to the player's ideas and help them to determine their efficacy. Sometimes the idea they've come up with isn't possible in the scenario you've laid out, and you need to guide them to find the knowledge that will set them on the right path. Alternatively the solution they've presented could be totally viable, but you just hadn't considered or prepared for it. In that case you should do you best to accommodate the new strategy, whatever that may be, and make adjustments to the story accordingly.
Sometimes, your players will decide to change or abandon the path you've laid for them. If they've found it too difficult or discouraging, it is your job as the GM to attempt to give them renewed hope. If they decide that they don't want to complete the objective, perhaps you need to review the character's motivation and make changes that get them rededicated to the task.
Ultimately, sometimes your player's will just take the story in a totally different direction that you did not anticipate. It's important as a GM to be open to such changes (ultimately, it is the character's decision as to whether they will continue on the adventure path or not). Think ahead about what will happen if the players do not complete the path you set them on, and be prepared to relate to them the consequences. This means also being prepared to take the adventure in a new and different direction if the PCs decide to do so. Try to be supportive of the new direction, but also show the PCs the consequences of failing to complete the adventure as intended.