I think maybe the most important thing people taught me when I was starting out, was the absolute importance of "tone of voice". It was emphasized that first contact with clients and casting directors and everyone in ones world use tone of voice to figure out what we mean and if they want to engage with us. Words are important of course, but they are only a small part of the communication at best.
We naturally want to tip a personable server in a restaurant more because it contributes to our experience. It's the same when a salesperson is helping us that seems present and engaged; we are more likely to make a purchase.
The same principle applies not only to doing great voiceover work, but also to getting voiceover work. People hire the people that they like the best. The most important talking you do occurs before the job is even offered. Always take time to listen to new clients needs and to make sure the conversations are centered on them. Being easy company that is well spoken with a tone of voice that is attentive can do a lot to improves the number of jobs one gets.