Every voice actor will at one point or another hit a wall while recording a voice-over. A place where the energy doesn't deliver, where you just sound dull and monotone.
If you dream of getting to a place in your voice-over career where you are so good at voiceovers that you only have to do one take to get it perfect, then keep dreaming. I have never met a voice actor who can do that, despite having years of voice-over training and professional experience!
Knowing that this will happen, the key is to keep a bag of tricks on hand that can pull the energy out of your voice when you yourself have none. Such A Voice Coach & Producer Jody Petersen's answer to this dilemma is, "What?"
This voice-over technique works wonders if you are having trouble nailing the billboard. Let's say that you landed an LG voice over job, and you're having an off day -- for some reason you're having trouble hitting "LG: Life's Good."
Instead of getting frustrated and banging your head against the wall, try voicing the script and the billboard, followed by "What?" and then voice the billboard again. So it will go like this:
"blah blah blah. LG: Life's Good. What? LG: Life's Good."
Nine time out of ten, the second time you read "LG: Life's Good" you will deliver the goods! Just make sure to edit out "What?" after you get your good take.