My two cents...

When I meet someone I ask if they have flown in a small aircraft before.
Next I ask them if they have any troubles with motion sickness (boats,
planes, etc.). I explain that while most people don't have any issues
with motionsickness, it can happen. Then go on to the usual briefing and
you should have some idea as to how suceptible they may be to getting
sick on the flight.

Once they are in the glider and strapped in I've always explained to
them that I want to make them as comfortable on THEIR flight as possible
and if they should happen to start feeling "off" that the best thing is
for them to TELL ME if they are not feeling well. If that happens, I'd
rather bring them back to the gliderport rather than have them get sick.
I then point out the ziplock and explain "just in case it comes on
suddenly".

During the flight it's important, based on their level of flying
experience, to be sure to tell them what's happening and it's not a good
idea to do any serious thermalling until you've found out if you've got
a trooper or not. A sudden turn, etc. if they are anxious is a good way
to make a benign situation bad. Anyone who's done a lot of rides
probably has a story of the person who showes up for a glider ride with
1000+ hours in power who "never gets sick" and a couple cored turns in a
thermal and they are sweating bullets.

With all that said, pay attention to how much they are talking and ask
them how they're doing, particularly after your first few gentle turns
thermalling.

If they start feeling poorly, level out, shallow turn toward the
gliderport, GENTLY ease out the spoilers, and take 'em home. If it
starts, then it is highly unlikely that they'll feel better so best plan
is to get them on the ground because no one is likely to ever return
after an intro ride if they actually get sick. They are your customer
and it's important to keep that in mind.

If they are a little green when you land, you can explain that with the
more exposure they get, typically the motion sickness will go away.

Gary
(8 years of rides, zero pukes)