I often compose the shot, taking care of all focusing and metering before asking someone to take the picture. Then it's relatively easy! You just hand someone the camera, show them the shutter release, and say "Stand here". My girlfriend's 11-year old daughter has taken some pictures with my cameras this way, as well as the occasional passing tourist. This is especially fun with the Polaroid land packfilm cameras so everyone else who's used to the instant gratification of digital and iPhones can see what they took.
I do the 'stand here' too! But between my intention to use larger aperture and the volunteer's effort trying to add her/his own artistic touch it's 50-50 possibility to nail the shot.