regardless of musical abilities, i find them performing best if they are fed proper monitors. some like to hear themselves loud, some like them just a tad beneath everyone else. some guitarists and vocalists like the airconditioning down because their fingers/vocal cords seem to tighten up in a cold room.
i think it's best to explain the recording process and strategy to be used before the beginning of a session to make them understand that they are allowed to make mistakes as everyone can easily be punched-in these days (yes, even drums sometimes). this makes them comfy somehow. it helps to orient them on how much time will be allocated to each recording process, and show them how goals can be acheived given their particular budget.
it pays to encourage clients regardless of their abilities, and assure them that making mistakes in the studio is normal. i find that adding unnecessary pressure to a recording musician only aggrevates his situation or elicit a more "careful" performance, rather than a "bold" one.
incidentally, i have been in the hot seat of recording in the late 90's as a drummer. it felt absolutely horrible when my band mates pressured me to play "faster" (so much faster in fact than what we have practiced). since it was a three-peice band and both agreed that my playing was too "slow" i had no choice but to really push myself to satisfy their imagined tempo. i counted every second, every beat, hardly making any fills anymore. it was an overall bad experience. when we got home and listened to the tape, the tempo was almost a quarter faster than the real tempo of the song! it was way too fast that they laughed. i didn't find it the least bit funny. so whenever i see a particular band-mate giving the guy in the hot seat a difficult time, i tell him nicely to back-off a bit.
if a recording musician keeps making more mistakes than normal, i request for a reschedule of the session.
and lastly i schedule the recording of different parts separetly whenever possible. this ensures that every band member comes in fresh and enthused to lay down his parts. in addition, it is easier to pick out mistakes if the sessions are exclusively dedicated.