It took me a long time to figure out because many of the outlining techniques I tried didn't help. In fact often after I wrote the outline I would become bored with the story and move onto something else.
Then I found the Snowflake Method, created by Randy Ingermanson. There are a total of ten steps, but I only use the first five in my personal outline method.
Step one is the one sentence summary of the novel. Describe the main character (MC) by their distinguishing traits, not a name, and try to keep the whole sentence to less than 20 words. Randy says that less than 15 is ideal, but I've only been able to do that once!
Step two expands the first sentence into a five sentence paragraph. The first sentence should describe the main character and setting. The second, third, and fourth start with a description of an action that the main character takes, and then the resulting disaster. The last sentence should describe the end of the story. I will often skimp on the last sentence since I don't always know how the story will end. The important part is to know the tone of the ending, so that it can be portrayed throughout the whole novel, so sometimes the last sentence is as simple as, "happy ending", "sad ending", or "everyone dies".
The third step gets easier; one page of character descriptions including all of the main characters. I think the most important part is figuring out what each character wants, why they want it, and how the wants will change the plot. Randy's website specifies a number of other things that one can consider adding to the character profile, but as a writer gains more experience they always seem to make their own.
Step four involves the expansion of the one paragraph summary into a one page summary. This should include the actions of the MC's, the resulting disasters, and the MC's emotional reaction to that. I have major problems knowing when minor characters should be introduced, so I try to figure that out in this step.
Step five is to create one page of description for each major character and a half page for each of the minor characters. I've found that the important part here is to really expand on the minor characters wants, and making sure they are working on those wants throughout the book.
After that I deviate from the method by writing a bullet point outline for the novel. I'll start with the disasters and then work backwards, stating all information that needs to be given to the reader before the ending, and work until I find the beginning of the novel.
Then I start writing!