Very broad question - and not really in the right area - but I'll give you a couple of thoughts.
1. As a rule a large diaphram condensor mic, works best on vocals (there are way too many mics to offer one - and diffeerent voices work better with different mics)
2. Subtle compression to control peaks can help, bu nothing is better than a singer who "works the mic".
3. Placing the mic slightly above the singer, pointing down (about to the bridge of the nose) can help prevent the plosives (pops from p sounds, etc). This also gets the singer to look up (vs. looking down) which helps to open up the throat)
4. A good engineer rides the faders (rather than depending on the compressor). So make sure you understand the material - having a lyric sheet so you can discuss speific parts of the song (or make notes where plosives may be, etc).
5. A vocal booth, or a controlled area (corner with sond treatment, etc) can control or at least limit the reflections that can cause vocals to sound "harsh". Unless you have a great sounding room, you are better off recording the vocal flat and then adding reverb/delay as needed.
6. However, the vocalists often like to hear some reverb/delay in the headphones - this not only helps the vocalist (since they sound "better" with the echo) but good vocalists work with the echo as part of the performance.
7. In addition to providing echo in the phones, make certain the vocalist gets exactly what they need in the headphones - whatever they need to give a good performance
8. Make sure you provide a comfortanle setting for the vocalist. Things that can help are: provide some beverage (warm tea or room temp water are good - soda, alcohol, iced liquids, etc - are all bad for the throat), provide someplace for lyric sheets that can be eaily seen. Provide decent light, but not too much light (many singers, like mood lighting.
9. Turst you ears - listen for pitch problems, articulation, etc. If you are the engineer, point out concerns to the producer. If you are the producer, help the singer understand what is wrong (but always stay positive).
10. Make sure the singer understands that you are there to help them. If they make an error they should stop and try again (rather than going through the whole song) - unless you have plenty of recording media and good editing capabilities. Make it enjoyable for the singer (less stress = better performance)
11. Always have the record button on - you never know when the perfect take will happen (often the 1st take or the "warm-up" turns out to be the best.