The link to the threads on the singing voice contain most of my comments on vocal health and links to medical websites on voice disorders and vocal health, so I won't go into repeating myself too much here. I recommend that anyone serious about singing and mainting their voice read that information if they have questions. I will add this thread to that collection.
I haven't had time for Mudcat much in the last few weeks (and now I have THREE tapes I need to make and send out for jobs, so I still don't have my Mudcat song recorded....). Anyway, I will repeat here that singing is a physical performance, and voice teachers often remind their students that they have to train physically and continue ongoing maintenance to keep the voice in shape, just like an athlete. Have you ever started a physical workout routine and then stopped? Obviously you lose some of the strength, endurance, ability, skills, that you don't continue working out. It is the same with the voice. If you don't sing something that stretches your full range on a regular basis, then you lose the top and bottom notes of your range that you achieved and have to start working again to regain them. If you abuse your vocal folds and lungs with smoke or drink alcohol, don't get enough sleep, or if you scream, talk too much in a loud voice, take medications that dry up your sinus or throat, etc, then you pay the price with how it affects your singing voice.
Some basic preventive measures are:
1. Learn good singing technique and practice it regularly.
2. Don't strain your vocal folds by yelling, talking too much, or pushing your voice out of your range.
3. Live a healthy lifestyle (eat right, sleep well, drink water, avoid smoke and too much alcohol).
4. Rest your voice when you are sick. Hum for practice.
5. Humidify your home in dry climates, especially when you are sleeping.
Those are some basics. Prevention is easier than cure, as with most things.