Recording vocals is a gentle process, not only for artist, but for recording engineer too. There are several things that can go wrong. The things that I mention here are based just on my opinion and experience and someone might disagree or say they are clichés. Commenting is desirable!
Vocal Recording Mistakes, Avoid These
1. Not to be encouraging and polite to artist
This applies specially when you have your first recording session with the artist. There is usually some uncertainty in the air when starting a recording session and it is very important to get a relaxed atmosphere in the recording studio. So pay attention to your approach.
2. Poor acoustics of recording room or booth
The more dry you can make your room acoustically, the better. If you have to record in untreated room, use mic screen, reflexion filter, what ever you want to call it. It doesn´t cost too much considering you are serious about recording good vocals.
3. Use of too much sound processing
I wouldn´t recommend any use of equalizer while recording because it´s always final. You can´t go back but you can always use it later. It is reasonable to use low cut in your mic or desk and slight compressing and mild de-essing, if available. Compressing is recommendable because it allows you to record higher signal levels and thus make better AD-conversion.
4. Bad microphone
There are microphones and microphones. I wouldn´t claim that everything that is cheap is poor quality but there is certain relation between price and quality. Maybe you can find a bargain but I personally wouldn´t buy a microphone without testing it unless it is renowned for its quality.
5. Bad cue mix
If your artist doesn´t hear clearly his or her own voice, there can´t be good results. Equally bad thing is that the vocal channel is mixed too loud. Yes, it is you who have to ask the artist, if the cue mix is good or not. Some singers prefer singing with reverb some do not. It is your job to ask and make it as good as possible.