Agree with MissMonicaE's answer. If you feel you need to change the sequence of events "just to mix things up", then your story was boring. Now it is boring and confusing. All other things being equal, people prefer linear storytelling.
For example, Tolkien skillfully uses nonlinear storytelling in Lord of the Rings, but only because he must -- he often has 3 (or 4!) separate groups doing stuff at the same time. Rather than referring to dates (that the reader won't remember), he refers to events that all his groups experience. Occasionally the narrator reminds the reader what the other groups are doing. Or, characters from one group will fill in other characters on what their group was doing. (So the reader gets both narrative, self-commentary, and interaction, all in one passage.)
Of course, other writers do this. Victor Hugo, Charles Dickens, Shakespeare. It's a classic technique. Even Homer does it.
Summary: Flashbacks are great, but only use them when they serve a distinct purpose, because they DO make it harder on your reader.