Have your pipes gotten a little rusty? For singers, your body is your instrument, so always warm up before practicing or performing—simple scales and arpeggios sung to different vowels and other sounds are good for gradually extending your range as your muscles warm up.
If you're out of practice it's important not to do too much too soon as you can easily strain your throat and vocal chords. Short sessions are a good idea to begin with.
Make sure you're standing properly as a good posture will help you produce a better sound - avoid slouching or raising your shoulders and be as relaxed and natural as possible.
If you feel tension creeping in do some shoulder exercises to help you relax.
Breathing is very important for singers, so make sure you breathe from the correct part of your lungs, and work on strengthening your diaphragm to give you lots of breath control.
Listen hard to yourself at all times - are you perfectly in tune? Is your enunciation clear? Think about rhythm and phrasing too.
Every singer benefits from regular practice but how much you do and what you do depends on the kind of singing you prefer - it might be worth practicing with a microphone for instance.
For more, or to get started restoring your voice to what it once was, watch this video lesson!