There are really two questions here.
1. How do you write a stand alone story?
Writing a good stand-alone story is not the same as writing the first novel in a series. A stand-alone story requires a satisfying ending that resolves all of the important questions in the plot. Your story should not require a sequel to complete it. You are not writing the first part of a series. You are writing a stand-alone novel. You are not writing The Fellowship Of The Ring, you are writing The Hobbit.It might be useful to think of the opposite of a stand alone story. In the TV show, The Flash, each episode has a story. Many of the episodes are linked together, but they always (especially early episodes in the series) wrap up with the bad guy being defeated by The Flash. The problem is presented, explored, and then resolved. They start out as stand-alone, or episodic stories. They have an ending. But as the episodes go on, you start to have story elements that tie between two or more episodes. Captain Cold doesn't get caught, and so we know there is more story. Not just because a bad guy is on the loose, but because we see a scene showing Captain Cold getting a new partner to fight The Flash. "Hook" scenes like this clue the audience in to the fact that there is more to the story.
If Captain Cold had just gotten away, we could have accepted that as the end of the story. Untidy endings are pretty compelling sometimes. But because we see Captain Cold planning to fight The Flash, we know that it's not really the end.
Books that are a part of a series use "hooks" like this to carry the reader through multiple plot arcs. Sometimes the overarching story actually plays out over more than one book. The Fellowship Of The Ring ended with the fellowship reaching the river where Boromir dies. It was a satisfying end to their flight from the shire and across/under the mountains, but we knew the story wasn't over. They hadn't completed their quest. It's a good book, but it wasn't stand-alone.
So, to create a stand-alone, you need to be ready to tell your whole story. Don't get caught up in writing clues about future books or adventures. If you do have something you plan on using later, make sure that it doesn't make the reader expect another book. If the emperor is going to be resurrected, let the heroes be satisfied that they defeated him. You can hint toward a "magic stone" that has the power to restore life, but don't say, "As soon as the emperor dies, his second in command will just bring him back to life." If you do, the reader will know that there is more to this particular story. A stand-alone is not afraid to end. Think of a movie rather than a TV show.
2. How do you create sequel potential?
Now this is where the answer gets fun. Short answer: make your world big enough to have more than one story.Long answer, well as Delia Sherman says in the episode, any good story has sequel potential. Let's consider some types of sequels. Necessary Sequels - These are the sequels that are required to complete the plot arc of the first book. This is the book that someone picks up knowing before hand that they are getting into a series. A reader must be ready to commit to this story, and they can't pick it up in the middle because they'd be totally lost no matter how good the writing is.
- Perpetual Sequels - I stopped watching Arrow, and countless other american style TV shows because they never reached a satisfying ending. They aren't planning on ending until the ratings drop. LOST had this problem. These stories avoid answering any question completely. Instead they keep stringing you along by "showing" not "telling" and make you think that every situation is important. Soap Operas also use this technique. Most modern TV dramas do as well.
- Episodic Sequels - These are sequels that include the same main characters, and events from one book/episode will carry over into the next. But the reader won't need to know the first story at all to enjoy the second. Star Trek: The Next Generation is a great example of this storytelling method. They didn't have the luxury of knowing that their audience would buy whole seasons on VHS, DVD, and then watch them all on Netflix. Every episode had to be enough to attract a new fan who could pick up that episode and then start watching from there.
- World Based Sequels - These are sequels that take place in the same world, but don't necessarily include the same characters. If you liked Star Trek: The Next Generation, chances are that you gave Deep Space Nine and Voyager a shot.
Next, let's look at some of the reasons people will want to read another book.
- Character - Let's face it, if you get to know a character, and you like them, you'll probably want to read about them again.
- World - If you have created a great world, then people will want to read more about it. Think of Terry Pratchett's Discworld.
- Relationships - This is similar to characters, but if you have a relationship that your readers want to see more of, they'll ask for another book even if there isn't one planned.
- Persistent Conflict - If there is a bigger conflict that your story takes place in, then your readers will want to find out more. If your character's parents died mysteriously as part of their backstory, readers will want to find out about that, but since it's backstory they won't expect it to be resolved in the first novel.
Here are some other tools you can use.
- Job - If your character has a job that will present opportunities for drama/conflict, then readers will probably be willing to check out another book. Superheroes have villains. Doctors always have new patients. Detectives will have new crimes to solve.
- Backstory - Give your character a backstory that is unresolved.
- Conflict - create a persistent conflict. Your character has an arch enemy who he puts in jail (e.g. the joker) or who flies off in a space ship (e.g. Darth Vader). Your hero saves the day for one town, but the bandits are now one town over.
- Exploration - This could fall into the Job category, but Dr. Who is a time traveling explorer. He has great episodic stories, but we always know he's going to explore something new next time.
Be careful not to make your first story dependent on future story arcs. Don't set "hooks", but give Easter Eggs. When a reader reads your second novel, you want them to be glad they read the first, but the first book has to be solid on its own.
In the end, any good story has sequel potential even if the sequel takes place 2,000 years later after the end of civilization. You just have to make your world big enough to hold it.