In penning down thoughts in a story, it's important to keep them close to the heart of your character. Maintain a consistent voice for your character – if your protagonist is a flamboyant dreamer, make sure their thoughts reflect the same.
Similarly, a pragmatic character would have more logical, grounded thoughts. Thought exposition is a great tool to delve deeper into their psyche. It ain't all about exposition though, subtlety is the key! Use indirect characterization, like their reactions and choices, to show what your character is thinking.
Lastly, do make sure to use italics where thoughts are directly reported. Let the readers get a ticket to a rollercoaster ride inside your character's mind!