Musicians who aren't engineers feel like they can do your job better than you. Because of this most of the guitarists and drummers you work with with bitch and moan about doing a million takes.
The best way I find to deal with this is by telling them these things:
1. "if you want it to sound bad, I can leave it. Fine by me. Just don't put my name on it."
2. "think of it as practicing. By the time you're done, you'll be better at playing your song"
3. "get out."
4. "that was good, but you're better than that"
5. "try it again, but this time play it like you own a pair"
I can add many more.
But the idea is that you want them to trust that what you do is for their benefit while at the same time making them feel comfortable with recording you by saying that their takes aren't going to cut it in a way that will keep them entertained.