This almost became a "listacle", and I'm not a fan of listacles. Granted, sometimes there is just no way around one, but let's make an effort to not make this post one of them...
My job depends on my voice. I don't necessarily mean the voice acting. My day job is in a place where I am on the phone a quarter of the day and when I am on the phone or dealing with people face to face, I need to have a fair bit of authority, if friendly authority, present in my voice.
Sounding half dead or nasal, or even like I don't have a voice, doesn't lend to the image of being that authority.
But how to do that?
What to do:
Keep hydrated. This almost goes without saying. Letting yourself go dry will result in no voice at all, or a throat that feels like a thousand razors were shoved down it.
Stay active. I know, I know - this piece of advice comes up each and every time I write an advice piece on how to improve anything. However, it's absolutely true. Your body isn't a bunch of disconnected bits and pieces. It's a machine that needs each piece to run as well as they can or the whole works comes to a screeching halt. A sound body is a sound mind and a sound spirit... and your voice will sound better. Exercise and an active lifestyle will also increase your lung capacity and that leads to the next point...
Remember to breathe when speaking. Your larynx requires air to work. With no breath, you have no voice. This is the primary reason having excellent control over your breathing (hence #2 being so important!) is critical to maintaining your voice. Script writers also have a nasty tendency to write really long sentences and expect the reader to accommodate. While you can take a tiny breath on a comma, don't count of them to always be there.
Avoid the cold. You know that shock you feel when you first step out into the cold? Your throat, and by extension your voice, also feels that same shock. Not to mention that cold air is typically very dry air, which is also your voice's #1 Enemy. If you can't avoid the cold, you can mitigate the impact by wearing insulation, such as a scarf or by doing your jacket all the way up (or both). Stay warm. Drink tea or something to soothe the throat.
Get plenty of rest. Like staying active, this may seem odd but your body needs plenty of rest to repair and recoup from the stresses of your day to day.
Avoid caffeine. I know I said drink tea, but I mean tea that doesn't have a whole lot of caffeine. As a coffee hound, it's very hard to follow my own advice here. I live on the stuff... and by extension that means I live on caffeine. Outside of too much caffeine preventing you from getting rest (especially if you indulge in it after lunch) when it's bedtime, caffeine also is a dehydrator which takes us right back up to our first point of staying hydrated. If you drink too much of something that dehydrates you, then you're shooting yourself in the foot.
Practice. I read out loud every single chance I get. Full, projected, volume. I also run a fan based radio station for our alliance in EVE Online, which also requires voice work as a radio personality. This does two things - first is that I get to practice, and hear myself, in a relaxed and fun environment. I also read which makes me practice reading for audiobooks and learning to pace myself while having an appreciative senior listen and give feedback... even if the feedback is a thank you.
Avoid overly dry air in your home (and workplace). Dry air, like cold air, is a huge problem for your throat and voice. If the air is too dry, it will suck the hydration right out of your throat and dry it out. This will then kill your voice faster than anything. If you find that your home and/or workplace is too dry (and you can get away with this), invest in a small humidifier and add some humidity. Not too much or you'll have mold (and that will be a whole other set of issues!), but just enough to dispel the instant desert sensation. Ironically, dry air isn't great for your belongings either as it leads to static which can kill electronics just as quickly as it can kill your voice.
Eat a healthy diet. Like exercise and a healthy lifestyle, what you put into your body will also help your voice and throat. Not to mention you'll just be healthier... and possibly live longer.
What not to do:
Drink alcohol. Alcohol, like caffeine, will dry out your throat and your voice. It also changes the natural tone of your vocal cords. You can indulge, but with moderation and keep hydrated with water in between each drink.
Smoke/vape. Smoking will kill you. It's a proven fact. And even if it wasn't, smoke damages your throat by hitting it with chemicals and smoke that dries out your larynx. Some people in the voice business swear by it because it adds a raspy tone but you can train that tone in and get rid of it when you don't want it. Once you gain a smoke related rasp you can never get rid of it. It's permanent. Vaping, on the other hand, opens a whole new can of worms. Before there were cases of "popcorn lungs", there is also the point of what vaping does. You're adding vapour into your lungs, as well as water, and that is like causing a case of pneumonia or bronchitis. Either one of these means being out of business for weeks, if not months. And, if you give yourself popcorn lung, that could mean permanently.
Force yourself to be heard in noisy environments (or other things that could lead to voice strain). What happens when you strain a muscle lifting too much or overstretching? The exact same thing can happen to your voice if you force yourself to be heard in a noisy environment. If you absolutely have to be heard, use a microphone and listen to your body. If you feel like your throat is dry, or worse yet sore, that's the first clue that something is up. Stop what you're doing or risk not having a voice for a few days to a few weeks.
Drink plenty of water (and keep more on hand). Taking care of your voice doesn't stop outside of the studio. It continues. Always keep water close by and drink plenty of water beforehand.
Don't eat a heavy meal right before the session. I wouldn't go hungry, but the problem with a big, huge meal is that stuff gets stuck. You're still digesting. If you have to eat, eat something light, easily digested, and will satisfy you. That "Hangry" feeling is also counter-productive. If you have to eat, or get caught after a big dinner with someone special, brush your teeth, go for a short, light walk (this helps with digestion), drink some tea or lemon water, and give yourself a bit of time before heading in.
Do light vocal exercises. I know a few people who find this step silly, but it's like going to the gym without a warm up. You will be putting your voice through some serious paces... warm it up first.
Stay relaxed. Tensing up with change the pitch of your voice, the volume, and your breathing will be off. You just won't sound right. If you're nervous, give yourself a moment to chill out.
Stand up. It's easier to get a full lung full of air while standing. If you can't stand, sit up straight, but maintain a relaxed but ergonomic posture in the chair.
Read the script first. Do I really have to remind you to be prepared? Also, in reading the script (or book) first gives you insight on tone, pacing, and what it is you're conveying.
Remember your audience. Reading an audiobook for a technical or scholarly audience will be significantly different than one for fiction, and again if your audience is younger or older. If you are voice acting, you also have to take into account the character. Audiobooks are more complex as not only do you have to be the narrator, but sometimes (if it calls for it) an entire cast of characters.... and this is precisely why we read the script or book first.
This is all true whether you work in a call centre (can't be on the phones without a voice, can you?), the radio, voice act or are a narrator for audiobooks. Your voice is your income. If you don't treat it properly (and the body behind it), it will leave you out in the cold.
I'm speaking from experience.