Listen to your client
First and foremost, listen to your client. Listen to the directions they are giving you for the script. Normally a client will tell you the overall tone and many times will have you record a quick sample to see if that is what they are looking for. Always ask questions if you aren't sure. If they don't give any directions, then ask, even after reading the script and you're not sure what kind of style they want. A lot of times you will read a script and the content is self explanatory. If you are doing a commercial for a local company and they aren't sure what kind of styles they want then send them a few samples to choose from.
Listen to names and pronunciations
Many times I will get scripts that have names or places I have never heard of or don't know how to pronounce. I always look up the name or word to double check to make sure I pronounce it correctly. Sometimes when it's a foreign language I will look it up on YouTube or Google Translate and listen to it over and over again until I master the correct pronunciation.
Listen to your voice
If you have listened to your client and the style that they want, then next is to listen to your own voice. What does is sound like? Does it fit what the client wants? EVERYTIME I record something I listen to what I have recorded even if I did a perfect read because I have to make sure my voice or style fits what the client wants.
Listen to Others
The best way to learn voice over styles is by listening to others in the voice over business. Listen to their different styles and how they read a script. I'm always listening to commercials on TV, radio or mobile apps. I ask myself this question. How is their delivery and why was that style chosen?