Have you ever thought of what your protagonist’s (or other characters’) favorite music genre or even favorite song might be? Think about how old s/he is and what type of music was popular when they were in their teens & early 20s.
This is particularly useful in character development since it helps define the person. For example, if someone prefers Beethoven to acid rock it clearly tells you something about them. When you’re getting to know a character yourself, listening to “their” favorite music an help you get in their zone as well. When your character comes to life it makes writing so much easier.
This is also a way for readers to relate to a character, especially if they share a favorite song with someone in your book. Just be aware that you can’t include lyrics of a song without permission, but the title works if it’s a popular song with which most of your fans are familiar.