Have a new story to tell.
If you haven't planned out your overall story as a series from the beginning (that is, you deliberately set it up to be three, five, seven, etc. books), and you're just writing an additional story with the same characters, then make sure you have a reason to write something about them.
Your sequel should have a beginning, middle, and end. It should be a standalone story (even if you need the previous story to understand it, which isn't the same thing). It should have a reason for existing.
It should not be a rehash of the first story.
To take a TV example, the show Heroes did very well in its first year because it was very fresh and new and was telling new stories, despite uneven writing. Creator Tim Kring thought that people enjoyed the idea of "characters slowly discovering they have powers" and created a new batch of people slowly discovering they have powers at the beginning of Season 2. However, what people were really enjoying was the crazy breakneck pace of the end of Season 1, where the characters had finished discovering and were now doing. So Season 2 was, in a word, dreadful. Kring was trying to write Season 1 over again.
Contrast that with the movie How to Train Your Dragon and the sequel. The first movie is about Hiccup and Toothless learning to trust one another and being able to work together to end the Viking-Dragon war. But the sequel is set five years later. There's already peace; they don't re-fight that battle. Hiccup and Toothless face dragon trappers, a dragon-kidnapping vigilante, and a crazy war leader. There has been significant character development for everyone.
So whatever story you told in your first book, make your sequel something completely different. If it was a Hero's Journey, the new one can be a family drama. If your first book was an origin story, your sequel can be your newly minted hero/ine(s) learning how to use his/her/their gifts, alone or in concert, on a regular basis, and how that fits into everyday life — what happens after the debut. If your first one is a romcom, the second can be a political battle.