Remember how the first movie ended. The climax and falling action should probably tie in to the plot of the sequel. In fact, the falling action may very well set up the plot of the sequel.
Craft a whole new storyline. Obviously, the story might be a tad similar to the first because it revolves around the same characters and theme, but make sure the direct external conflict and the surrounding events are different.
Develop a new internal conflict for at least one, if not more, of the characters. This drives the story as much as external conflict. However, most internal conflicts are resolved in the first movie and it's very difficult to create new ones.
Add at least one new character to the main cast. The audiences always want to see a different person, and adding a new personality can help create your movie's new conflicts, both external and internal.
Keep in mind what made the first movie such a success. This is the one thing you do want to make sure is also part of your sequel. Here's a hint: it's usually the personalities of the characters.