To add to Donkey and Pete's wisdom.
1. Singing in pitch is more inportant than how cool your voice sounds. I don't care if you can sing like Ronnie James Dio or Marvin Gaye, if you are singing flat it sounds like crap. If you're singing backup, people are going to hear the note more than whether or not it sounds like Janis Joplin.
2. Come in at the right time. If the guitar player messes up the band can easily recover. Not so when the singer or drummer does. If a singer comes in a measure early or late, the song structure is messed up and the groove has to be interupted to fix it.
3. Learn how to use the mic. You don't have to eat the mic in order to be heard. Screaming into a mic, while it's right up to your mouth is a great way to piss off a soundman or damage speakers. Plus, you can't articulate if a friggin mic is in your mouth. In general, give your self enough space from the mic to properly move your mouth. The louder, or more forcefully you sing the more you should back away from the mic. If you want to accent the bass in your voice, sing closer. If you want to accent the highs, back off.
4. What you hear in the monitors isn't exactly what the audience is hearing. Just because the stage monitors don't reveal your rich baritone voice, doesn't mean that the Mains and Subwoofers aren't. Just sing the right pitch and let the soundman worry about it.
5. Sing from your chest, not your head/throat. That throaty voice might sound cool but it's a good way to trash your voice. Using your chest improves endurance, projection, and pitch. The vocal chords are like the strings on a bass. Your breath are your fingers plucking it. Your sinus cavities, mouth, diaphragm, abdomen are like the body of the bass. They add to the resonance of the strings.
6. Develop your ear to develop your voice. Develop your voice to develop your ear. The ear and voice complement each other. There are no fret markers on a voice. You have to go with your ear. You have to somehow hear a note to sing a note. You'll find that as you develop your voice, your ear will improve because of the practice and the synergy with the voice.
7. Listen to your body. If you sing an E, pay attention to how your throat and other parts of your body feel. Remember that for the next time you sing an E. Practice doing that and next thing you know, it becomes natural. Being able to feel the notes is key when the stage sound isn't cooperating with you.
8. Find "your" voice. Find the voice that you are most comfortable and capable of singing. Not everyone can sing as high as Steve Perry or as low as Barry White. Not everyoe sings as smooth as Frank Sinatra or as raspy as Janis Joplin. Find your range and your tone and work with it.
9. Use your bass to help your singing. Play a C on the bass and sing it, paying special attention to matching the pitch, and how your throat feels when you hit the pitch. Play a scale on the bass and sing the scale, paying attention to how your throat and body feel when going from the root to the II, etc. Play a major arpeggio on your bass and sing it, paying attention to how you feel when you go from the root to the third, third to fifth, fifth to octave. Play a note on the bass and sing the major third. Play the major third on the bass, if you're having trouble hearing it. Try all the above with different scales, keys, arpeggios, and chord intervals.