Whether it's a commercial or narration project, I always try to pull a personal experience out of my performance. This should be something you're passionate about. Something happy, funny, or sad that happened to you.
It's easy to pick up a script and read it, thinking your voice is good enough to make the message sound genuine and plausible. The talents who book are the ones who do the research and study the message in the copy before recording it. Get as familiar with the product or business as possible. What is the marketing objective of the message? Are there specific words in the scripts that need to be more accentuated than others?