Write an outline of your story. The outline doesn't need to include every plot twist, but it should at least cover the main high points of the story. Consider drawing a graph where you mark the rising and falling action of your story. You should find that rising action and points of tension outweigh the lulls in your story.
Write a first draft of your story based on your outline. Pay attention to the rising and falling action. Allow your story to breathe. If you don't find the tension levels peaking near the end of each chapter of your book, work on timing them better. The end of one chapter should encourage readers to move to the next.
Integrate conflict into your story. The conflict could be internal or external, meaning within your character (emotional) or some outside force (physical). Conflict doesn't need to be action oriented. It just needs to be something that captures reader interest and makes them want to find out more. Waiting for a character to discover a secret can be written in a way that excites the reader.
Build anticipation. Always keep something in play your readers want to find out. It could be the answer to a question or exposing another character's betrayal. Readers always need to be anticipating what comes next.