You know how authors release novellas or shorts between novels in a series? I tried this "extras" technique out on two WiPs, and it's crazy how much it helped. The trick is to write them before you pen a word of your first draft.
All you need to do is take your protagonist back to a point in his or her life that will be important to the plot later. Doesn't have to be pivotal, as long as they discuss or think about it in the actual manuscript.
Get your pen/computer/typewriter out, and write the scene. You'll have a better understanding of your character and their motivations, as well as setting, the relationship with other characters, and backstory. Makes a huge difference.