My heroine's goal was simple. She wanted the jury to find her innocent. This is something that can happen (or not) in a few short days. Goals that take years to accomplish don't work in a novella. The challenge I had was keeping the time short—and the romance long, if you know what I mean. I didn't want the relationship to feel rushed.
Readers want to know what makes your characters tick and why they do the things they do. Goals and motivations must be clear.
Save the introspection and flashbacks for your novels.
The fewer characters in a novella, the better. Everyone has to pull his or her weight and then some.
Stick to the main story as much as possible. Subplots should be short and easily resolved. Some people say you should tell your story from a single viewpoint, but I write romance and like to go back and forth between the hero and heroine. Two viewpoints are probably the most you can get away with.
Remember, big ideas don't need a lot of words—they just need the right words. And that's the long and the short of it.