If possible, stand when you record your script. Good posture will help you maintain good breath support, making it easier to speak in a strong, clear voice. Especially when recording energetic commercials or video game characters. If you're unable to stand, do your best to sit up straight. It helps!
Make sure that you stay hydrated. Room temperature bottled water works very well. However, if your mouth gets dry, noises will become an issue. Smacks, clicks and pops can be challenging to remove in post-production. Plus, when you can hear smacks and clicks, it can distract the performance.
Warming up your voice
You can find all kinds of crazy tongue-twisters online but I like to keep things simple by reading through the alphabet a few times and purposely over-articulating each letter. Each letter will give your lips, tongue and vocal cords an excellent workout before the project.
Pre-reading the script
It's a good idea to pre-read the script to find the right combination of volume, projection and pacing for the voiceover. But don't practice it profusely before the session. You'll get too comfortable with how YOU see the script being performed, but the client may have different ideas about how they want the script performed. It's best to read it over a couple of times and then wait for the client's performance instructions.
Voice acting isn't just reading into a microphone
When we talk to people, we use facial expressions and hand gestures. We don't typically do this when we read to ourselves, but I would recommend it while you're recording your script. It will bring a conversational tone to the performance. Most commercials these days require a more genuine read.
Read from a printed copy of the script as well. Don't hold the script while you record. You don't want to risk the sound of rustling papers in the recording. A music stand works well for this. As you read through the script, keep a pencil handy and make marks to remind you when to breathe, pause, and pitch your voice lower or higher for emphasis. The goal is to make your voiceover sound natural, comfortable, and appropriate for the commercial or narration. When you make a mistake in your reading, pause for a couple of seconds. Then, back up a sentence or two, and begin reading again. Those pauses will give you a great visual cue when editing your voiceover for broadcast.