Douglas - in 40 years I've had only 4 throwup in x000 flights. I personally don't tell them where the sick bags are kept, if we've got any!

Perhaps a bit formal - but here goes - a preflight briefing with open questions keeps them involved and it enables you to assess their state both visually and in terms of their understanding of the potential of their flight (perhaps ability to comprehend gliding practice). I explain that if they want to handle the aircraft controls, then there are three introductory exercises (lookout, use of elevator to control attitude and speed, and ailerons to roll the aircraft) as they will be demonstrated and attempted. Use "your patter" on the ground with them strapped in the cockpilt. In the air you will demonstrate with patter, they will follow through on the controls and their attempt is all we are aiming to achieve. They will be impressed with what they can do!

It's useful to watch and talk through previous launches with them and any approaches and landings.

Cockpit checks, I get them to follow through with right hand on the stick to aquaint them with elevator sense fore and aft, aileron operation left and right and continue with "I have control""we won't move the stick that far in the air" I then complete the control checks for rudder and full & free.
Continue to involve them in the ongoing cockpit check ... instruments "mine are set, don't worry about yours"... "The green knob on the ... is the trim on the elevator, full & free movement, set for .... launch, and if the cable breaks, it's set for landing" ..... "The blue lever is the airbrakes, fully open L and R (get them to look L & R) flush and locked". Two seperate movements.

This complies with the CAA requirement to avoid the (anti) Hire & Reward sentiments of providing instruction.

Anticipate the launch proceding and give a verbal commentary.
Launches and landings are stressful for you and very much so for them - so talking during these phases of flight are essential.

In the air smooth movements (as airline pilots do normally), if possible plan your flying so that every turn an instructional turn - it's much easier for you, and it maximinses their handling and exercise time. Flight planning needs to aim to achieve this.

Shallow angles of bank in turns and get them to follow through even though they don't want to fly the aircraft. You will be able to sense if they are tense, leading or lagging on the controls.
This helps them compute what their behind senses and their eys see. So long as their brain can keep rationalising these two information flows they won't throw up.
Perhaps a little more speed will make the aircraft more stable for them.

Something I try to do is to be enthusiastic about how well they are attempting ...... . Words like "that's good!" Even when they're attempts are aweful but safe - keep the words positive - "Try that again! This time, concentrate on .....or try ....". Don't be afraid to re-demonstrate the excercise.

It's essential to keep up a conversation, (keep them occupied to prevent nerves developing) at a pace they can absorbe and comply with your requests to lookout, move controls and give you control on request.
Not answering questions (can you see ... and not looking means overload), complying with look R to see ... or the back of their neck trembling and a halo of steam above their head indicates they're working at peak performance and perhaps they are about to ...... At any rate they are past learning.
Mental fatigue will set in before phyical fatigue, but you will notice erratic control movements indicating perhaps mental fatigue is present, even after a few minutes. A minute or two off the controls and they may be ready for another try. Limit first flights to say 20 mins max.
Perhaps this is how you do it now and it's more a question of what they've just had to eat. however, I hope there's a few clues for you Douglas - best of luck!