yep i did this in my salon after it had been open a year,
we had sealed envelopes with the questionaire in them together with a s a e back to the salon, they didnt have to leave their names but a lot did, they were given an envelope as the paid at the till, it was all explained,
sort of questions that were asked were,,,,
what service did you have at the salon today,?
who was your stylist/tech?
did you find the stylist/tech helpful knowledgeable/professional etc
what was the best part of your visit to our salon?
what was the worst thing about your visit?
were you kept waiting for your appt
were you offered refreshments
all sort of things like that , loads of questions
and the response we got was excellent
some were funny, some got us thinking about the layout of our salon etc
one point we did learn from was that no one knew what out juniors were called, wich i thought was a shame, so now we actually state their names to the customers when we hand them over to the juniors
hth x