Your on but you feel off:
I have been a singer since before I started high school. I have wrapped my throat in scarves, sipped ginger tea, refused to scream on roller coasters and declared "vocal rest" at the most inopportune moments. The life of the singer revolves around protecting your voice, babying your voice and driving your non-singing friends crazy. I have found that my vocal chords and my throat are like a canary in a cave. If they don't feel right, a cold, flu or other general cootie is generally right around the corner. Does this send panic through me as I set out to sing or act? Yes it does. Does the same issue hold true for public speaking? Yes it does.
How do we take care of our voice when we don't feel quite up to par?
Six Tips for Babying Your Voice
Otherwise known as singers' tea, you can use the store bought ginger bags or make your own. I've even made this out of powdered ginger in a pinch. For fresh, slice a couple of pieces of peeled ginger and steep in water for about 10 minutes over a low flame. Add honey and lemon to taste. The tea will sooth your throat and bring down inflammation.
I use this as an emergency technique. If my chords are swollen, I hum almost sub-vocally. It's very quiet -- just air over the chords at any pitch. This thins the chords out and allows them to be used without them being swollen. If your chord are really swollen you may have to postpone your speech. Laryngitis needs rest. Don't push it.
Get enough sleep. If you didn't get sleep the night before try to get at the very least some quiet time where you don't talk and can sit with your eyes closed. Better yet try to get a nap in. If none of that is possible then find a quiet place to sit for a few minutes. The reboot will do you good. Breathe deeply and center yourself.
Watch the coffee and alcohol intake:
Caffeine can dry out the throat so watch those triple Americanos before you go in. Alcohol is also drying so if you have a concern about your throat, don't drink the night before. I know many performers who won't drink at all before a presentation or performance. Think of this as one of the few things can control. You can't control climate, germs, sleep (sometimes), or the number of people you might have to talk to before your presentation. You can however control what you consume.
I like to call this the surefire path to blowing it big. Practice is a delicate art. You should practice. You should not practice excessively. Specifically you should not speak your speech over and over and over again the morning you are speaking. Know your topic. Practice your topic. The morning of your topic treat yourself well, know you are ready and baby your voice a little.
Although the voice can be a delicate instrument there is no reason to panic. Most likely your voice will be fine all the way through your presentation. Just remember to respect it and baby it a little.