In Voiceover, imagination is a key skill. So many things have to come from your imagination. Who are you and who are you talking to in any script? What is the situation and where are you? What just happened before the script begins? There is much to create to get yourself to just the right mental state to have the emotion you need for a good performance. All this comes from your imagination. There is a great VO saying I love: If you can see it, they can see it. So how do you see it, in as much detail as possible?
I think building a strong imagination is not unlike building muscle or cardio fitness. You've got to use it, consistently. So daydream. Did a teacher ever tell you you'd get nowhere in life daydreaming in class? Well, now daydreaming is the KEY to your career. Take that! So imagine something, anything. In as much detail as you can. What does is look like, feel like, smell like, taste like? You're on a beach. Your Skiing. You're a rock star! It doesn't matter, you can go anywhere and be anything. It's fun! Get lost in it. Remember to engage your senses. The smells, the sounds, ect. Your in a kitchen baking cookies with your grandma and you can actually smell the cookies! How does it make you feel? What do you say to her?
Let's say you have a script that calls for a warm, comforting voice? Imagining this cooking scene could put you in just the right mental space for a great read. And it will be easier because you'll actually be feeling the feelings of love and comfort. The more you use your imagination the better you will get and the stronger you can engage your imagination, the stronger, more authentic feelings you can evoke. You can add to this by using your body. If I need to feel angry or aggravated, I'll stomp my feet, through my hand around and start swearing. But I am also imagining a situation where someone has done me wrong and I am so angry about it.
Another thing that helps the imagination is meditation. Meditation helps you clear your mind of distraction so you can really go there on your imaginary adventures. It also calms the body which gets rid of nervous energy and puts you more in control of the body.
Bonus tip! Improvisation is another form of acting that is very useful in Voiceover. Here is a simple improv exercise you can do at home anytime. Take an object, any object. Your cell phone for example. Look at it, hold it. Start coming up with other things that it could be. Go as fast as you can. It doesn't have to be practical. It can be silly, crazy, whatever. Stand it up straight and it's a monolith. Lay it down flat. It's an ice skating rink. Flip it around and around length wise. It's a paddle boat wheel and so on. It doesn't even matter if it doesn't make complete sense. The key is to quickly engage your imagination for new ideas. You'll get better with practice so pick a new object every day.