Sometimes the best way to come up with the next big idea is to study the big ideas that others have had already. Pick an everyday object that makes your life more convenient -- or more difficult -- and study its history. Understanding how life was without it, how it used to work and what ideas people have had to improve it so far may inspire you with a way to take it to the next level.
Many improvements in life come from a new way of looking at things. If you can create an invention that helps people see things in a new way, it may lead to further improvements. An invention as simple as a gauge that people can attach to their faucets to measure how much water they use to wash the dishes or brush their teeth could permanently alter people's lifestyles.
If you're really stuck, start asking the people around you what improvements their lives could use. Remember that any inventor who is trying to make the world a better place has to start by finding a way the world isn't as good as it could be. Ask people what tools in their lives are frustrating to use or what processes take far too much time or effort. Those problems could inspire you to create a brand new solution.
To come at things from the opposite angle, figure out what you love to do and find a way to do it more and better. If you like sketching, try inventing a pencil that never needs to be sharpened. If you like skateboarding, you could invent a skateboard with a smoother ride or better control, or protective pads that fit comfortably under your clothes. What you love is usually something you're good at. That passion and experience will give you a leg up in creating a worthwhile invention.