So your manuscript is going along great, and suddenly you have an idea. Why not plan for a sequel? A prequel? Even a trilogy? Isn't that what all the publishers are looking for these days?
Since I've been pondering this question with my own manuscript, I thought I'd check to see what the conventional wisdom is on the topic.
First off, some positives and negatives for the author:
On the plus side: You get to use the same storyworld you've already spent so much time working on. You don't have to dream up new characters, they're all ready to go into new adventures. You can make use of some of that backstory you've been dying to reveal. Your readers (who liked the first book) will probably stick with you for the next.
On the minus side: You use the same storyworld and characters you've spent so much time on (story fatigue anybody?). You've got to come up with realistic adventures once the big climax is finished in the first book. Multiple books have individual and overall arcs. If you haven't kept that in mind, you may not have planted enough information in the first book to take advantage of in the third. In addition, some readers just don't like them.
Think about J.K. Rowling. There were props and incidents in the first book that weren't fully revealed until the last one. That took some serious planning.
For those of us awaiting publication, it's fine to make notes for a sequel (some publishers are more anxious for them than others), but it's better to hold off on the actual writing of a sequel. Once you've made a sale, you can talk about subsequent books. Of course, if you're self-publishing, you get to make the decision.
So here's what different authors have to say about what they like and don't like about sequels, and a few tips and tricks to keep in mind.
Nicola Morgan, from Help! I Need a Publisher! has a very comprehensive post on the topic.
Veronica Roth shares her two biggest challenges when working on a sequel.
Janice Hardy gives us the trouble with triples: writing trilogies.
Tina Donahue lists some of the negatives she sees with sequels.
And Michelle Knudsen wrote a three-part series on sequels (would that be a trilogy?). Here's part 1, part 2, and part 3.
Maybe the trick is to be prepared for a series, be committed to a stand-alone, and let the readers decide if they want more. What do you think?