By the way, I recommend the book "Digital Portrait Photography" by Steve Sint. Very readable and pleasant presentation style, I found. I borrowed mine from the Toa Payoh Library. Should be returning it soon.
It depends on your budget and on what you are taking and working distance.
Typically lenses used are :
50/1.4 or 50/1.8
85/1.4 or 85/1.8
135/2 or 135/2.8
Longer focal lengths, you need to stand further away from subject.
You also need a higher shutter speed of 1/focal_length for longer lenses.
These are important/practical considerations other than just shallow DOF.
Also, I find that with a very shallow DOF, focusing becomes an issue. The slightest jerk will cause your subject to go out of focus. So even though at the right shutter speed you won't see a motion blur, your subject is still not properly focussed. And also, if the subject is has complex depths and curves, getting the right focus requires a tripod and lots of tweaking of focus and aperture sizes.