You might be surprised to learn that the quality of the headphone mix can often determine whether a singer gives a good performance in the studio. Even top-notch, highly experienced vocalists need a good monitoring reference when tracking. Some singers are more fussy than others, but the more comfortable they are while singing, the better.
If a vocalist has to think about what he or she is hearing, that will surely have a negative effect on their performance. If the singer is doing anything other than singing, you've lost. Without having the proper monitoring, the elements of pitch, emotion, and energy will suffer, and self-consciousness and loss of focus will likely result.
So the first step is to determine what the singer needs in the mix. If you have 48 tracks of instruments, densely arranged with all kinds of potentially distracting sounds, it can be difficult to create a good cue mix. Sometimes the arrangement for the recording is more complicated than what the singer is used to. This is frequently the case with singer-songwriters who are venturing into the studio for the first time. They're often baffled by singing along with drums, bass, guitars, strings, layers of synths, and percussion. Remember, just because it's on the multitrack doesn't mean the singer needs to hear it.
When it comes to the headphone mix, less is more. You should strive to build a mix that features only the essentials, keeping the "sweetening" elements out unless the singer asks for them or they're necessary to elevate the singer's emotional intensity. It's also vital to keep the mix clean enough to give the vocalist a solid pitch reference. A muddy headphone mix can blur the singer's sense of pitch. If a singer is having intonation trouble, I listen for elements that might be causing the pitch confusion. By pulling out an instrument or two, you can often clean up the mix and make life better for everyone.
Another important factor is how loud the singer's own voice will be relative to the rest of the mix. Whether due to vanity, hearing impairment, or just force of habit, many singers like their voices to be incredibly loud. Others prefer them lower. Regardless, the first thing to do is ask! Don't guess or do it the way the previous singer liked it. Ask what they want and give it to them. The same goes for reverb. Some singers want lots, others none, and others might want delay. Again, ask and make it so-it's always time well spent.
Some inexperienced singers have no idea what works for them in the headphones. Occasionally, I have to play games like varying the vocal level in their cans over the course of a take to see if the performance gets better one way or another. You need to do whatever it takes to get the singer singing and not thinking.
Another detail is the headphones themselves. Some singers carry their own or know from past experience what make and model they like. If possible, find out beforehand and get a set. At minimum, make sure you have a couple of different brands and models available at the session.